Fluent for Firefox Developers

This tutorial is intended for Firefox engineers already familiar with the previous localization systems offered by Gecko - DTD and StringBundle - and assumes prior experience with those systems.

For a more hands-on tutorial of understanding Fluent from the ground up, try following the Fluent DOMLocalization Tutorial, which provides some background on how Fluent works and walks you through creating a basic web project from scratch that uses Fluent for localization.

Using Fluent in Gecko

Fluent is a modern localization system introduced into the Gecko platform with a focus on quality, performance, maintenance and completeness.

The legacy DTD system is deprecated, and Fluent should be used where possible.

Getting a Review

If you work on any patch that touches FTL files, you’ll need to get a review from fluent-reviewers. There’s a Herald hook that automatically sets that group as a blocking reviewer.

Guidelines for the review process are available here.

To lighten the burden on reviewers, please take a moment to review some best practices before submitting your patch for review.

Major Benefits

Fluent ties tightly into the domain of internationalization through Unicode, CLDR and ICU.

More specifically, the most observable benefits for each group of consumers are


  • Support for XUL, XHTML, HTML, Web Components, React, JS, Python and Rust

  • Strings are available in a single, unified localization context available for both DOM and runtime code

  • Full internationalization (i18n) support: date and time formatting, number formatting, plurals, genders etc.

  • Strong focus on declarative API via DOM attributes

  • Extensible with custom formatters, Mozilla-specific APIs etc.

  • Separation of concerns: localization details, and the added complexity of some languages, don’t leak onto the source code and are no concern for developers

  • Compound messages link a single translation unit to a single UI element

  • DOM Overlays allow for localization of DOM fragments

  • Simplified build system model

  • No need for pre-processing instructions

  • Support for pseudolocalization

Product Quality

  • A robust, multilevel, error fallback system prevents XML errors and runtime errors

  • Simplified l10n API reduces the amount of l10n specific code and resulting bugs

  • Runtime localization allows for dynamic language changes and updates over-the-air

  • DOM Overlays increase localization security

Fluent Translation List - FTL

Fluent introduces a file format designed specifically for easy readability and the localization features offered by the system.

At first glance the format is a simple key-value store. It may look like this:

home-page-header = Home Page

# The label of a button opening a new tab
new-tab-open = Open New Tab

But the FTL file format is significantly more powerful and the additional features quickly add up. In order to familiarize yourself with the basic features, consider reading through the Fluent Syntax Guide to understand a more complex example like:

### These messages correspond to security and privacy user interface.
### Please choose simple and non-threatening language when localizing
### to help user feel in control when interacting with the UI.

## General Section

-brand-short-name = Firefox
    .gender = masculine

pref-pane =
    .title =
        { PLATFORM() ->
            [windows] Options
           *[other] Preferences
    .accesskey = C

# Variables:
#   $tabCount (Number) - number of container tabs to be closed
containers-disable-alert-ok-button =
    { $tabCount ->
        [one] Close { $tabCount } Container Tab
       *[other] Close { $tabCount } Container Tabs

update-application-info =
    You are using { -brand-short-name } Version: { $version }.
    Please read the <a>privacy policy</a>.

The above, of course, is a particular selection of complex strings intended to exemplify the new features and concepts introduced by Fluent.


While in Fluent it’s possible to use both lowercase and uppercase characters in message identifiers, the naming convention in Gecko is to use lowercase and hyphens, avoiding CamelCase and underscores. For example, allow-button should be preferred to allow_button or allowButton, unless there are technically constraints – like identifiers generated at run-time from external sources – that make this impractical.

In order to ensure the quality of the output, a lot of checks and tooling is part of the build system. Pontoon, the main localization tool used to translate Firefox, also supports Fluent and its features to help localizers in their work.

Social Contract

Fluent uses the concept of a social contract between developer and localizers. This contract is established by the selection of a unique identifier, called l10n-id, which carries a promise of being used in a particular place to carry a particular meaning.

The use of unique identifiers is shared with legacy localization systems in Firefox.


An important part of the contract is that the developer commits to treat the localization output as opaque. That means that no concatenations, replacements or splitting should happen after the translation is completed to generate the desired output.

In return, localizers enter the social contract by promising to provide an accurate and clean translation of the messages that match the request.

In Fluent, the developer is not to be bothered with inner logic and complexity that the localization will use to construct the response. Whether declensions or other variant selection techniques are used is up to a localizer and their particular translation. From the developer perspective, Fluent returns a final string to be presented to the user, with no l10n logic required in the running code.

Markup Localization

To localize an element in Fluent, the developer adds a new message to an FTL file and then has to associate an l10n-id with the element by defining a data-l10n-id attribute:

<h1 data-l10n-id="home-page-header" />

<button data-l10n-id="pref-pane" />

Fluent will take care of the rest, populating the element with the message value in its content and all localizable attributes if defined.

The developer provides only a single message to localize the whole element, including the value and selected attributes.

The value can be a whole fragment of DOM:

<p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'>
  <a data-l10n-name="privacy-url" href="http://www.mozilla.org/privacy" />
-brand-short-name = Firefox
update-application-info =
    You are using { -brand-short-name } Version: { $version }.
    Please read the <a data-l10n-name="privacy-url">privacy policy</a>.

Fluent will overlay the translation onto the source fragment preserving attributes like class and href from the source and adding translations for the elements inside. The resulting localized content will look like this:

<p data-l10n-id="update-application-info" data-l10n-args='{"version": "60.0"}'">
  You are using Firefox Version: 60.0.
  Please read the <a href="http://www.mozilla.org/privacy">privacy policy</a>.

This operation is sanitized, and Fluent takes care of selecting which elements and attributes can be safely provided by the localization. The list of allowed elements and attributes is maintained by the W3C, and if the developer needs to allow for localization of additional attributes, they can allow them using data-l10n-attrs list:

<label data-l10n-id="search-input" data-l10n-attrs="style" />

The above example adds an attribute style to be allowed on this particular label element.

External Arguments

Notice in the previous example the attribute data-l10n-args, which is a JSON object storing variables exposed by the developer to the localizer.

This is the main channel for the developer to provide additional variables to be used in the localization.

It’s worth noting that, when the l10n-args are set in the runtime code, they are in fact encoded as JSON and stored together with l10n-id as an attribute of the element.

Runtime Localization

In almost every case the JS runtime code will operate on a particular document, either XUL, XHTML or HTML.

If the document has its markup already localized, then Fluent exposes a new attribute on the document element - document.l10n.

This property is an object of type DOMLocalization which maintains the main localization context for this document and exposes it to runtime code as well.

With a focus on declarative localization, the primary method of localization is to alter the localization attributes in the DOM. Fluent provides a method to facilitate this:

document.l10n.setAttributes(element, "new-panel-header");

This will set the data-l10n-id on the element and translate it before the next animation frame.

This API can be used to set both the ID and the arguments at the same time.

document.l10n.setAttributes(element, "containers-disable-alert-ok-button", {
  tabCount: 5

If only the arguments need to be updated, then it’s possible to use the setArgs method.

document.l10n.setArgs(element, {
  tabCount: 5

On debug builds if the Fluent arguments are not provided, then Firefox will crash. This is done so that these errors are caught in CI. On rare occasions it may be necessary to work around this crash by providing a blank string as an argument value.

Non-Markup Localization

In rare cases, when the runtime code needs to retrieve the translation and not apply it onto the DOM, Fluent provides an API to retrieve it:

let [ msg ] = await document.l10n.formatValues([
  {id: "remove-containers-description"}


This model is heavily discouraged and should be used only in cases where the DOM annotation is not possible.


This API is available as asynchronous. In case of Firefox, the only non-DOM localizable calls are used where the output goes to a third-party like Bluetooth, Notifications etc. All those cases should already be asynchronous. If you can’t avoid synchronous access, you can use mozILocalization.formatMessagesSync with synchronous IO.


The majority of internationalization issues are implicitly handled by Fluent without any additional requirement. Full Unicode support, bidirectionality, and correct number formatting work without any action required from either developer or localizer.

document.l10n.setAttributes(element, "welcome-message", {
  userName: "اليسع",
  count: 5

A message like this localized to American English will correctly wrap the user name in directionality marks, allowing the layout engine to determine how to display the bidirectional text.

On the other hand, the same message localized to Arabic will use the Eastern Arabic numeral for number “5”.

Plural Rules

The most common localization feature is the ability to provide different variants of the same string depending on plural categories. Fluent ties into the Unicode CLDR standard called Plural Rules.

In order to allow localizers to use it, all the developer has to do is to pass an external argument number:

document.l10n.setAttributes(element, "unread-warning", { unreadCount: 5 });

Localizers can use the argument to build a multi variant message if their language requires that:

unread-warning =
    { $unreadCount ->
        [one] You have { $unreadCount } unread message
       *[other] You have { $unreadCount } unread messages

If the variant selection is performed based on a number, Fluent matches that number against literal numbers as well as its plural category.

If the given translation doesn’t need pluralization for the string (for example Japanese often will not), the localizer can replace it with:

unread-warning = You have { $unreadCount } unread messages

and the message will preserve the social contract.

One additional feature is that the localizer can further improve the message by specifying variants for particular values:

unread-warning =
    { $unreadCount ->
        [0] You have no unread messages
        [1] You have one unread message
       *[other] You have { $unreadCount } unread messages

The advantage here is that per-locale choices don’t leak onto the source code and the developer is not affected.


There is an important distinction between a variant keyed on plural category one and digit 1. Although in English the two are synonymous, in other languages category one may be used for other numbers. For example in Bosnian, category one is used for numbers like 1, 21, 31 and so on, and also for fractional numbers like 0.1.

Partially-formatted variables

When it comes to formatting data, Fluent allows the developer to provide a set of parameters for the formatter, and the localizer can fine tune some of them. This technique is called partially-formatted variables.

For example, when formatting a date, the developer can just pass a JS Date object, but its default formatting will be pretty expressive. In most cases, the developer may want to use some of the Intl.DateTimeFormat options to select the default representation of the date in string:

document.l10n.formatValue("welcome-message", {
startDate: FluentDateTime(new Date(), {
    year: "numeric",
    month: "long",
    day: "numeric"
welcome-message = Your session will start date: { $startDate }

In most cases, that will be enough and the date would get formatted in the current Firefox as February 28, 2018.

But if in some other locale the string would get too long, the localizer can fine tune the options as well:

welcome-message = Początek Twojej sesji: { DATETIME($startDate, month: "short") }

This will adjust the length of the month token in the message to short and get formatted in Polish as 28 lut 2018.

At the moment Fluent supports two formatters that match JS Intl API counterparts:

With time more formatters will be added. Also, this feature is not exposed to setAttributes at this point, as that serializes to JSON.

Registering New L10n Files

Fluent uses a wildcard statement, packaging all localization resources into their component’s /localization/ directory.

That means that, if a new file is added to a component of Firefox already covered by Fluent like browser, it’s enough to add the new file to the repository in a path like browser/locales/en-US/browser/component/file.ftl, and the toolchain will package it into browser/localization/browser/component/file.ftl.

At runtime Firefox uses a special registry for all localization data. It will register the browser’s /localization/ directory and make all files inside it available to be referenced.

To make the document localized using Fluent, all the developer has to do is add localizable resources for Fluent API to use:

<link rel="localization" href="branding/brand.ftl"/>
<link rel="localization" href="browser/preferences/preferences.ftl"/>

The URI provided to the <link/> element are relative paths within the localization system.

Custom Localizations

The above method creates a single localization context per document. In almost all scenarios that’s sufficient.

In rare edge cases where the developer needs to fetch additional resources, or the same resources in another language, it is possible to create additional Localization object manually using the Localization class:

const myL10n = new Localization([

let [isDefaultMsg, isNotDefaultMsg] =
  await myL10n.formatValues({id: "is-default"}, {id: "is-not-default"});


An example of a use case is the Preferences UI in Firefox, which uses the main context to localize the UI but also to build a search index.

It is common to build such search index both in a current language and additionally in English, since a lot of documentation and online help exist only in English.

A developer may create manually a new context with the same resources as the main one, but hardcode it to en-US and then build the search index using both contexts.

By default, all Localization contexts are asynchronous. It is possible to create a synchronous one by passing an sync = false argument to the constructor, or calling the SetIsSync(bool) method on the class.

const myL10n = new Localization([
], false);

let [isDefaultMsg, isNotDefaultMsg] =
  myL10n.formatValuesSync({id: "is-default"}, {id: "is-not-default"});

Synchronous contexts should be always avoided as they require synchronous I/O. If you think your use case requires a synchronous localization context, please consult Gecko, Performance and L10n Drivers teams.

Designing Localizable APIs

When designing localizable APIs, the most important rule is to resolve localization as late as possible. That means that instead of resolving strings somewhere deep in the codebase and then passing them on, or even caching, it is highly recommended to pass around l10n-id or [l10n-id, l10n-args] pairs until the top-most code resolves them or applies them onto the DOM element.


When writing tests that involve both I18n and L10n, the general rule is that result strings are opaque. That means that the developer should not assume any particular value and should never test against it.

In case of raw i18n the resolvedOptions method on all Intl.* formatters makes it relatively easy. In case of localization, the recommended way is to test that the code sets the right l10n-id/l10n-args attributes like this:


const l10nAttrs = document.l10n.getAttributes(element);

deepEquals(l10nAttrs, {
  id: "my-expected-id",
  args: {
    unreadCount: 5

If the code really has to test for particular values in the localized UI, it is always better to scan for a variable:




Testing against whole values is brittle and will break when we insert Unicode bidirectionality marks into the result string or adapt the output in other ways.

Manually Testing UI with Pseudolocalization

When working with a Fluent-backed UI, the developer gets a new tool to test their UI against several classes of problems.

Pseudolocalization is a mechanism which transforms messages on the fly, using specific logic to help emulate how the UI will look once it gets localized.

The three classes of potential problems that this can help with are:

  • Hardcoded strings.

    Turning on pseudolocalization should expose any strings that were left hardcoded in the source, since they won’t get transformed.

  • UI space not adapting to longer text.

    Many languages use longer strings than English. For example, German strings may be 30% longer (or more). Turning on pseudolocalization is a quick way to test how the layout handles such locales. Strings that don’t fit the space available are truncated and pseudolocalization can also help with detecting them.

  • Bidi adaptation.

    For many developers, testing the UI in right-to-left mode is hard. Pseudolocalization shows how a right-to-left locale will look like.

To turn on pseudolocalization, open the Browser Toolbox, click the three dot menu in the top right corner, and choose one of the following:

  • Enable “accented” locale - [Ȧȧƈƈḗḗƞŧḗḗḓ Ḗḗƞɠŀīīşħ]

    This strategy replaces all Latin characters with their accented equivalents, and duplicates some vowels to create roughly 30% longer strings. Strings are wrapped in markers (square brackets), which help with detecting truncation.

    This option sets the intl.l10n.pseudo pref to accented.

  • Enable bidi locale - ɥsıʅƃuƎ ıpıԐ

    This strategy replaces all Latin characters with their 180 degree rotated versions and enforces right to left text flow using Unicode UAX#9 Explicit Directional Embeddings. In this mode, the UI directionality will also be set to right-to-left.

    This option sets the intl.l10n.pseudo pref to bidi.

Testing other locales


For Firefox engineering work, you should prefer using pseudolocales. Especially on Nightly, localizations can be incomplete (as we add/remove localized content all the time) and cause confusing behaviour due to how fallback works.

Installing Nightly in a different locale

Localized Nightly builds are listed on the mozilla.org website.

Installing language packs on local builds

To fix bugs that only reproduce with a specific locale, you may need to run a development or nightly build with that locale. The UI language switcher in Settings is disabled by default on Nightly, because language packs can become incomplete and cause errors in the UI — there is no fallback to English for strings using legacy formats, like .properties.

However, if you really need to use this, you can:

  1. Open about:config and flip the intl.multilingual.enabled and intl.multilingual.liveReload preferences to true

  2. Open the FTP listing for langpacks and click the XPI file corresponding to your language and nightly version (note that, especially around merge days, multiple versions may be present).


    This is a Linux listing because that’s the platform on which we run the l10n jobs, but the XPIs should work on macOS and Windows as well. The only exception is the “special” Japanese-for-mac locale, which is in the mac/xpi subdirectory under latest-mozilla-central-l10n instead. (ja-JP-mac and ja will both “work” cross-platform, but use different terminology in some places.)

  3. Click through the prompts to install the language pack.

  4. Open the Firefox Settings UI.

  5. Switch to your chosen language.

Finding a regression in a localized build

You can run mozregression with localized builds!

At the commandline, if you wanted to find a regression in a Dutch (nl) build, you could run something like::

mozregression --app firefox-l10n --lang nl --good 2024-01-01

and that should run localized nightlies.

Inner Structure of Fluent

The inner structure of Fluent in Gecko is out of scope of this tutorial, but since the class and file names may show up during debugging or profiling, below is a list of major components, each with a corresponding file in /intl/l10n modules in Gecko.

For more hands-on experience with some of the concepts below, try following the Fluent DOMLocalization Tutorial, which provides some background on how Fluent works and walks you through creating a basic web project from scratch that uses Fluent for localization.


FluentBundle is the lowest level API. It’s fully synchronous, contains a parser for the FTL file format and a resolver for the logic. It is not meant to be used by consumers directly.

In the future we intend to offer this layer for standardization and it may become part of the mozIntl.* or even Intl.* API sets.

That part of the codebase is also the first that we’ll be looking to port to Rust.


Localization is a higher level API which uses FluentBundle internally but provides a full layer of compound message formatting and robust error fall-backing.

It is intended for use in runtime code and contains all fundamental localization methods.


DOMLocalization extends Localization with functionality to operate on HTML, XUL and the DOM directly including DOM Overlays and Mutation Observers.


DocumentL10n implements the DocumentL10n WebIDL API and allows Document to communicate with DOMLocalization.


DOM translation is asynchronous (e.g., setting a data-l10n-id attribute won’t immediately reflect the localized content in the DOM).

We expose a Document.hasPendingL10nMutations member that reflects whether there are any async operations pending. When they are finished, the L10nMutationsFinished event is fired on the document, so that chrome code can be certain all the async operations are done.


L10nRegistry is our resource management service. It maintains the state of resources packaged into the build and language packs, providing an asynchronous iterator of FluentBundle objects for a given locale set and resources that the Localization class uses.