Update system

Firefox's auto-update system downloads, verifies, and applies binary patches. The C++ updater process lives in toolkit/mozapps/update/; the patch format / metadata server contract is shared with the AUS (Application Update Service).

Components

toolkit/mozapps/update/
├── updater/                # C++ binary that applies the patch (own process)
├── common/                 # Shared C++ code
├── UpdateService.sys.mjs   # Front-end: schedules checks, fetches MAR, signals updater
├── UpdateLog.sys.mjs       # Logging
├── tests/
└── docs/

Plus:

• toolkit/mozapps/update/updater/ — the standalone updater binary.
• other-licenses/bsdiff/ — the bsdiff-derived patch algorithm.

How an update happens

1. UpdateService periodically checks aus5.mozilla.org (the production AUS) with the user's current version, build ID, channel, and OS.
2. AUS responds with a MAR (Mozilla ARchive) URL and a signature.
3. Firefox downloads the MAR (background, low priority).
4. After download, the updater process is spawned at the next quit/restart.
5. The updater:
   • Verifies the MAR signature against bundled certificates.
   • Applies the patch (file replace + bsdiff partials).
   • Restarts Firefox.

Channels

Firefox release channels are defined in defaults/pref/channel-prefs.js style files inside each app:

• nightly — daily builds.
• beta — pre-release.
• release — production.
• esr — Extended Support Release.

Related

• Reference / configuration
• Toolkit components