Firefox for Desktop

Desktop architecture

One of the keys to understanding how Firefox is architected is to know that Gecko is used to render both web content as well as the UI of the browser. This means that, to a large extent, the UI of Firefox is made using web technologies like HTML, CSS, and JavaScript. For example, the document chrome://browser/content/browser.xhtml is the markup for a single Firefox browser window. You can think of it almost like index.html for a webpage.

The decision to use Gecko for rendering the UI was made during an era when HTML was not yet expressive enough or sufficient to build desktop UIs. A different markup language, XUL, was developed for that purpose instead, which Gecko also knows how to interpret. Since that time, HTML and CSS has evolved to the point where XUL is not strictly necessary for most use-cases. Where possible, we have been gradually migrating our UI to use less XUL and more standard HTML and CSS.

Reusable UI Widgets

Many of the UI widgets in use in the user interface are built as web components that can be reused.

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The tabbrowser component manages the tabs in a browser window.

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Address Bar

The address bar (varyingly also referred to as the Awesome Bar, or Quantum Bar) handles users entering web addresses including a popup with suggestions from various sources including history, bookmarks and search engines.

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The places component includes the database that stores history and bookmarks as well as a set of UI components that present those to the user.

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Firefox Sync and Application Services

Application services includes the sync engines and Firefox Accounts integration.

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Developer Tools

The developer tools support web developers when building webpages but as the Firefox UI is itself built using web technologies it is also possible to use the developer tools to debug the Firefox UI.

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Browser Toolbox

The Browser Toolbox is a way to launch the developer tools in a separate process so that they can be used to debug the main browser UI.

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The Firefox Profiler is the way to measure the performance of your code. Go to to enable the button in Firefox and record a performance profile. These profiles can be uploaded and shared. It can identify slow parts of the code, and reveal the underlying behavior of how code runs.


The Windows installer for Firefox is built with NSIS. There are currently no installers for other operating systems.

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Application Update

Application Update is responsible for requesting available updates from the update servers, downloading them, verifying their integrity and then ultimately installing them. A binary patch tool (bsdiff) is used to reduce the size of update files and update files are delivered in the bespoke mar (Mozilla ARchive) format.

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