Toolkit components

toolkit/ is the shared layer between Firefox Desktop, GeckoView/Fenix, the standalone xpcshell, and various smaller tools. It contains UI widgets, components, modules, and platform integrations that are not specific to any one app.

Sub-areas

toolkit/
├── components/         # Mid-level components (a partial list below)
├── content/            # Shared chrome content (widgets, dialog templates)
├── modules/            # Shared JS modules (.sys.mjs)
├── mozapps/            # Larger app-level features (extensions UI, update, profile)
├── crashreporter/      # Crash reporter
├── library/            # libxul build target
├── locales/            # Toolkit-level localized strings
├── pluginproblem/      # Legacy plugin error UI
├── profile/            # Profile manager
├── recordreplay/       # Web Replay (R&D)
├── system/             # Per-platform integrations
├── themes/             # Common theme bits
└── xre/                # XULRunner / app startup glue (XRE = "XUL Runtime Environment")

Notable components

Component	Purpose
toolkit/components/places/	Bookmarks + history (Places SQLite)
toolkit/components/search/	Search service
toolkit/components/extensions/	WebExtensions runtime
toolkit/components/glean/	Glean SDK
toolkit/components/telemetry/	Legacy telemetry
toolkit/components/passwordmgr/	Password manager
toolkit/components/antitracking/	ETP plumbing
toolkit/components/captivedetect/	Captive portal detection
toolkit/components/contentanalysis/	Enterprise content analysis
toolkit/components/normandy/	Normandy / Nimbus experiments
toolkit/components/nimbus/	A/B experimentation client
toolkit/components/url-classifier/	(lower-level shim; main impl in netwerk)
toolkit/components/uniffi-bindgen-gecko-js/	UniFFI ↔ JS bindings generator
toolkit/components/pdfjs/	Vendored PDF.js
toolkit/components/reader/	Reader Mode

mozapps

toolkit/mozapps/ contains larger user-facing features:

• extensions/ — about:addons UI and AddonManager.
• update/ — auto-update front-end.
• defaultagent/ — the Windows "default browser agent".
• installer/, installer/windows/, installer/linux/ — installer pieces.
• preferences/ — about:preferences glue (some moves are happening into browser/).

Modules

.sys.mjs modules under toolkit/modules/ include Services.sys.mjs, AppConstants.sys.mjs, PromiseUtils.sys.mjs, Timer.sys.mjs, XPCOMUtils.sys.mjs, etc. — the JS standard library for chrome code.

Related

• Firefox Desktop
• GeckoView