Pocket Guide: Shipping Firefox

Estimated read time: 15min


The purpose of this document is to provide a high level understanding of how Mozilla ships Firefox. With the intention of helping new Mozillians (and those who would like a refresher) understand the basics of our release process, tools, common terms, and mechanisms employed in shipping Firefox to our users. Often this document will introduce a concept, explain how it fits into the process, and then provide a link to learn more if interested.

Repositories & Channels

Shipping Firefox follows a software release train model along 3 primary code repositories; mozilla-central (aka “m-c”), mozilla-beta, and mozilla-release. Each of these repositories are updated within a defined cadence and built into one of our Firefox products which are released through what is commonly referred to as Channels: Firefox Nightly, Firefox Beta, and Firefox Release.

Firefox Nightly offers access to the latest cutting edge features still under active development. Released every 12 hours with all the changes that have landed on mozilla-central for Desktop and on main in firefox-android for Android.

Every 4 weeks, we merge the code from mozilla-central to our mozilla-beta branch. For Android, we branch from main on firefox-android to a release branch. New code or features can be added to mozilla-beta outside of this 4 week cadence but will be required to land in mozilla-central and then be uplifted into mozilla-beta. Similarly for Android, uplifts are required to land in main on firefox-android before backporting to the firefox-android release branch.

Firefox Beta is for developers and early adopters who want to see and test what’s coming next in Firefox. We release a new Desktop/Android Beta version three times a week.


The first and second beta builds of a new cycle are shipped to a subset of our Beta population. The full Beta population gets updated starting with Beta 3 only.*

Each Beta cycle lasts a total of 4 weeks where a final build is validated by our QA and tagged for release into the mozilla-release branch for Desktop. On Android we release from the same release branch used during the Beta cycle.


Firefox Developer Edition is a separate product based on the mozilla-beta repo and is specifically tailored for Web Developers.

Firefox Release is released every 4 weeks and is the end result of our Beta cycle. This is our primary product shipping to hundreds of millions of users. While a release is live, interim updates (dot releases) are used to ship important bug fixes to users prior to the next major release. These can happen on an as-needed basis when there is an important-enough driver to do so (such as a critical bug severely impairing the usability of the product for some users). In order to provide better predictability, there is also a planned dot release scheduled for two weeks after the initial go-live for less-critical fixes and other ride-along fixes deemed low-risk enough to include.


Firefox ESR (Extended Support Release) is a separate product intended for Enterprise use. Major updates are rolled out once per year to maintain stability and predictability. ESR also contains a number of policy options not available in the standard Firefox Release. Minor updates are shipped in sync with the Firefox Release schedule for security and select quality fixes only.

Further Reading/Useful links:

Landing Code and Shipping Features

Mozillians (those employed by MoCo and the broader community) land lots of code in the Mozilla repositories: fixes, enhancements, compatibility, new features, etc. and is managed by Mercurial (aka hg). All new code is tracked in Bugzilla, reviewed in Phabricator, and then checked into the mozilla-central repository using Lando.


Some teams use GitHub during development but will still be required to use Phabricator (tracked in Bugzilla) to check their code into the mozilla-central hg repository.

The standard process for code to be delivered to our users is by ‘riding the trains’, meaning that it’s landed in mozilla-central where it waits for the next Beta cycle to begin. After merging to Beta the code will stabilize over a 4 week period (along with everything else that merged from mozilla-central). At the end of the beta cycle a release candidate (RC) build will be generated, tested thoroughly, and eventually become the next version of Firefox.

Further Reading/Useful links:

An exception to this process…

Not all code can simply wait for the normal train model to be included in a Firefox build. There are a variety of reasons for this; critical fixes, security concerns, stabilizing a feature that’s already in Beta, shipping high priority features faster, and so on.

In these situations an uplift can be requested to take a recent landing in mozilla-central and merge specific bits to another repository outside the standard train model. After the request is made within Bugzilla, Release Management will assess the potential risk and will make a decision on whether it’s accepted.

Further Reading/Useful links:

Ensuring build stability

Throughout the process of landing code in mozilla-central to riding the trains to Firefox Release, there are many milestones and quality checkpoints from a variety of teams. This process is designed to ensure a quality and compelling product will be consistently delivered to our users with each new version. See below for a distilled list of those milestones.



Day of Week

Merge Day

Nightly W1


Day 1 of the new Nightly Cycle

PI Request deadline

Nightly W1


Manual QA request deadline for high risk features

Feature technical documentation due

Nightly W2


Deadline for features requiring manual QA

Beta release notes draft

Nightly W4


Nightly features Go/No-Go decisions

Nightly W4


Feature Complete Milestone

Nightly W4


Last day to land risky patches and/or enable new features

Nightly soft code freeze start

Nightly W4


Stabilization period in preparation to merge to Beta

QA Test Plan approval due

Nightly W4


Last day to provide QA with feature Test Plan sign-offs

String freeze

Nightly W4


Modification or deletion of strings exposed to the end-users is not allowed

QA pre-merge regression testing completed

Nightly W4


Merge Day

Beta W1


Day 1 of the new Beta cycle

Pre-release sign off

Beta W3


Final round of QA testing prior to Release

Firefox RC week

Beta W4


Validating Release Candidate builds in preparation for the next Firefox Release

Release Notes ready

Beta W4


What’s new page ready

Beta W4


Firefox go-live @ 6am PT

Release W1


Day 1 of the new Firefox Release to 25% of Release users

Firefox Release bump to 100%

Release W1


Increase deployment of new Firefox Release to 100% of Release users

Scheduled dot release approval requests due

Release W2


All requests required by EOD

Scheduled dot release go-live

Release W3


By default, ships when ready. Specific time available upon request.

The Release Management team (aka “Relman”) monitors and enforces this process to protect the stability of Firefox. Each member of Relman rotates through end-to-end ownership of a given release cycle. The Relman owner of a cycle will focus on the overall release, blocker bugs, risks, backout rates, stability/crash reports, etc. Go here for a complete overview of the Relman Release Process Checklist.


While Relman will continually monitor the overall health of each Release it is the responsibility of the engineering organization to ensure the code they are landing is of high quality and the potential risks are understood. Every Release has an assigned Regression Engineering Owner (REO) to ensure a decision is made about each regression reported in the release.*

Further Reading/Useful links:

Enabling/Disabling code (Prefs)

Within Firefox we allow the ability to Enable/Disable bits of code or entire features using Preferences <preferences>. There are many reasons why this is useful. Here are some examples:

  • Continual development over multiple release cycles without exposing partially completed features to our users

  • Provide the ability to quickly disable a feature if there is a problem found during the release process

  • Control features which are experimental or not ready to be shown to a specific channel population (e.g. enabled for Beta but disabled for Release)

  • A/B testing via telemetry experiments


Normandy Pref Rollout is a feature that allows Mozilla to change the state of a preference for a targeted set of users, without deploying an update to Firefox. This is especially useful when conducting experiments or a gradual rollout of high risk features to our Release population.

Further Reading/Useful links:

Release & Feature QA

Release QA is performed regularly and throughout the Release Cycle. Organized in two-week sprints its primary goals are:

  • Qualifying builds for release

  • Feature testing

  • Product Integrity requests

  • Bug work

  • Community engagement

Features that can have significant impact and/or pose risk to the code base should be nominated for QA support by the feature owner in its intended release. This process is kicked off by filing a Product Integrity team request PI request. These are due by the end of week 2 of the Nightly cycle.


Manual QA testing is only required for features as they go through the Beta cycle. Nightly Feature testing is always optional.

Further Reading/Useful links:


As we deliver new features to our users we continually ask ourselves about the potential impacts, both positive and negative. In many new features we will run an experiment to gather data around these impacts. A simple definition of an experiment is a way to measure how a change to our product affects how people use it.

An experiment has three parts:

  1. A new feature that can be selectively enabled

  2. A group of users to test the new feature

  3. Telemetry to measure how people interact with the new feature

Experiments are managed by an in-house tool called Experimenter.

Further Reading/Useful links:


Approval Flag - A flag that represents a security approval or uplift request on a patch.

Bugzilla - Web-based general purpose bug tracking system and testing tool.

Channel - Development channels producing concurrent releases of Firefox for Windows, Mac, Linux, and Android.

Chemspill - Short for Chemical Spill. A chemspill is a rapid security-driven or critical stsbility dot release of our product.

Channel Meeting - A twice weekly time to check in on the status of the active releases with the release team.

Dot Release Drivers - Issues/Fixes that are significant enough to warrant a minor dot release to the Firefox Release Channel. Usually to fix a stability (top-crash) or Security (Chemspill) issue.

Early Beta - Beta releases with the features gated by EARLY_BETA_OR_EARLIER enabled. The first 2 weeks of Beta releases during the cycle are early beta releases.

Feature Owner - The person who is ultimately responsible for developing a high quality feature. This is typically an Engineering Manager or Product Manager.

Fenix - Also known as Firefox Preview is an all-new browser for Android based on GeckoView and Android Components

Github - Web-based version control and collaboration platform for software developers

GTB - Acronym for Go to build. Mostly used in the release schedule communication (“Go to build on March 18”), this means that we initiate the building of a specific release.

Landing - A general term used for when code is merged into a particular source code repository

Lando - Automated code lander for Mozilla. It is integrated with our Phabricator instance and can be used to land revisions to various repositories.

Mercurial - A source-code management tool (just like git) which allows users to keep track of changes to the source code locally and share their changes with others. It is also called hg.

Merge - General term used to describe the process of integrating and reconciling file changes within the mozilla repositories

Nightly Soft Code Freeze - Last week of the nightly cycle on mozilla-central just before the merge to beta during which landing risky or experimental code in the repository is discouraged.

Normandy - Normandy is a collection of servers, workflows, and Firefox components that enables Mozilla to remotely control Firefox clients in the wild based on precise criteria

Nucleus - Name of the internal application used by release managers to prepare and publish release notes. The data in this application is fetched by mozilla.org.

Orange - Also called flaky or intermittent tests. Describes a state when a test or a testsuite can intermittently fail.

Phabricator - Mozilla’s instance of the web-based software development collaboration tool suite. Read more about Phabricator as a product.

PI Request - Short for Product Integrity Request is a form submission request that’s used to engage the PI team for a variety of services. Most commonly used to request Feature QA it can also be used for Security, Fuzzing, Performance, and many other services.

Preferences - A preference is any value or defined behavior that can be set (e.g. enabled or disabled). Preference changes via user interface usually take effect immediately. The values are saved to the user’s Firefox profile on disk (in prefs.js).

Release Candidate - Beta version with potential to be a final product, which is ready to release unless significant bugs emerge.

RC Week - The week prior to release go-live is known as RC week. During this week an RC is produced and tested.

Release Cycle - The sum of stages of development and maturity for the Firefox Release Product.

Regression Engineering Owner - A partner for release management assigned to each release. They both keep a mental state of how we are doing and ensure a decision is made about each regression reported in the release. AKA REO.

Release engineering - Team primarily responsible for maintaining the build pipeline, the signature mechanisms, the update servers, etc. aka releng

Release Management - Team primarily responsible for the process of managing, planning, scheduling and controlling a software build through different stages and environments. aka relman.

Relnotes - Short for release notes. Firefox Nightly, Beta, and Release each ship with release notes.

Repository - a collection of stored data from existing databases merged into one so that it may be shared, analyzed or updated throughout an organization.

Ride Alongs - Bug fixes that are impacting release users but not considered severe enough to ship without an identified dot release driver.

Rollout - Shipping a release to a percentage of the release population.

Status Flags - A flag that represents the status of the bug with respect to a Firefox release.

String Freeze - Period during which the introduction, modification, or deletion of strings exposed to the end-users is not allowed so as to allow our localizers to translate our product.

taskcluster - Our execution framework to build, run tests on multiple operating system, hardware and cloud providers.

Telemetry - Firefox measures and collects non-personal information, such as performance, hardware, usage and customizations. This information is used by Mozilla to improve Firefox.

Train model - a form of software release schedule in which a number of distinct series of versioned software releases are released as a number of different “trains” on a regular schedule.

Tracking Flags - A Bugzilla flag that shows whether a bug is being investigated for possible resolution in a Firefox release. Bugs marked tracking-Firefox XX are bugs that must be resolved one way or another before a particular release ship.

Throttle/Unthrottle a rollout - Throttle is restricting a release rollout to 0% of the release population, users can still choose to update but are not updated automatically. Unthrottle is removing the release rollout restriction.

Uplift - the action of taking parts from a newer version of a software system (mozilla-central or mozilla-beta) and porting them to an older version of the same software (mozilla-beta, mozilla-release or ESR)