Firefox for Android (Fenix)

The modern Firefox for Android browser, codenamed Fenix, is a Kotlin app that embeds Gecko via GeckoView. It replaced the old Fennec app around 2020.

Purpose

Fenix is a native Android app with its own Compose / Fragment / Activity UI. It uses GeckoView for everything related to web content (rendering, navigation, JavaScript). The app's core is split between Mozilla Android Components (a library of reusable browser building blocks) and Fenix-specific UI/feature code.

Directory layout

mobile/android/
├── geckoview/                    # The embedding library (Kotlin + JNI to Gecko)
├── geckoview_example/            # A bare-bones example app using GeckoView
├── android-components/           # Reusable browser components (browser-engine, browser-toolbar, etc.)
├── fenix/                        # The Firefox for Android app
├── focus-android/                # Firefox Focus / Klar (privacy-oriented sibling)
├── exoplayer2/                   # Vendored ExoPlayer for media
├── components/                   # Misc shared Android components
├── gradle/                       # Gradle wrappers and configs
├── docs/
└── ...

Key directories

• mobile/android/fenix/app/src/main/java/org/mozilla/fenix/ — main Kotlin source for the app.
• mobile/android/android-components/components/ — modular Android Components (browser-state, browser-engine-gecko, browser-toolbar, concept-engine, …).
• mobile/android/fenix/app/build.gradle — app build config.
• Top-level build.gradle, settings.gradle, gradlew — the Gradle harness for the whole Android build.

Architecture

graph TD
    Activity[HomeActivity / BrowserActivity] --> Fragment[BrowserFragment]
    Fragment --> EngineView[GeckoEngineView]
    EngineView --> GVSession[GeckoSession]
    GVSession --> JNI[GeckoView JNI layer]
    JNI --> Gecko[Gecko Content Process]
    Fragment --> Toolbar[BrowserToolbar]
    Activity --> Store[BrowserStore<br/>Redux-like state]
    Activity --> ServiceWorkerR[Sync, Sync, Glean, Nimbus]

The Mozilla Android Components team maintains a unidirectional state architecture (BrowserStore and BrowserState) inspired by Redux. Fenix consumes this store and renders Composables / Views from it.

Build

The Android build is driven by Gradle but starts from mach:

ac_add_options --enable-application=mobile/android
./mach configure
./mach build
./mach package
./mach android-emulator   # optional: spin up an emulator
./mach gradle :fenix:app:assembleDebug

The resulting APK lives under <objdir>/gradle/.../fenix-*.apk.

Components used by Fenix

A non-exhaustive list of features that come from android-components/:

Component	Purpose
browser-engine-gecko	Bridge between concept-engine and GeckoView
browser-state / browser-store	Redux-style state
browser-toolbar	Address bar Composable
browser-tabstray	Tabs tray UI
browser-session-storage	Tab persistence
browser-icons	Site favicons
feature-tabs, feature-search, feature-session, feature-prompts, …	Reusable feature modules
service-glean, service-nimbus	Telemetry and experiments
service-firefox-accounts	Sign-in plumbing

Related

• GeckoView
• Process model
• Telemetry / Glean