Claude in Firefox

A Firefox WebExtension (shown in Firefox as Claude Browser Bridge, since add-on names may not contain the Firefox trademark) plus an MCP server that lets Claude Code (CLI and desktop app) drive Firefox the way the official Claude in Chrome extension drives Chrome: list tabs, navigate, click, type, scroll, take screenshots, read the page as an accessibility outline, fill forms, run JavaScript, and read console and network logs.

Anthropic ships no Firefox extension (its help centre says Claude in Chrome supports only Google Chrome). This project is an independent, clean-room implementation: it contains no Anthropic code or assets and never touches your Claude credentials. It is not the claude.ai side panel chat; it is the browser-automation half, exposed to Claude Code as tools named mcp__claude-in-firefox__*.

Tools

Coordinates are CSS pixels of the viewport and map 1:1 onto the screenshot the model just took (scale shrinks the image and the note says how to convert back).

Related MCP server: firefox-use

Architecture

Claude Code / desktop app
   │  MCP over stdio
   ▼
dist/mcp-server.js  ── JSON lines over a Unix socket ──▶  dist/native-host.js
(one per Claude session)       $XDG_RUNTIME_DIR/claude-in-firefox.sock   (spawned by Firefox,
                                                                           one per extension)
                                                                              │ native messaging
                                                                              │ (4-byte length + JSON)
                                                                              ▼
                                                                  extension background.js
                                                                  tabs · screenshots · console/network stores
                                                                              │ tabs.sendMessage
                                                                              ▼
                                                                  content.js in each page
                                                                  outline/refs · pointer · keyboard · forms · eval

The native host multiplexes any number of MCP servers onto the single extension connection, so several Claude Code sessions can share one Firefox. All wire types live in extension/src/shared/protocol.ts.

Install

Requirements: Firefox 128+, Node.js 22+, Claude Code.

git clone https://github.com/diegochagas/claude-in-firefox
cd claude-in-firefox
./scripts/setup

scripts/setup installs dependencies, builds dist/, writes the native-messaging manifest to ~/.mozilla/native-messaging-hosts/claude_in_firefox.json (pointing at this checkout) and registers the MCP server in Claude Code (user scope) as claude-in-firefox. Re-running it is safe. Pass --no-mcp to skip the Claude Code registration.

Then install the extension in Firefox, one of:

• Signed release (recommended): download the latest .xpi from Releases and open it in Firefox (about:addons → gear → Install Add-on From File, or drag it onto a window). It is signed by Mozilla through the unlisted channel, so it stays installed, and the manifest's update_url points at the release's updates.json, so Firefox pulls newer versions by itself.

• Try it now (temporary): npm run firefox opens your default profile with the extension loaded until Firefox quits, or use about:debugging#/runtime/this-firefox → Load Temporary Add-on → dist/extension/manifest.json.

The toolbar badge reads on when the native host is connected; clicking the icon opens a status popup (connection state, tabs Claude has driven, a Reconnect button). There is no chat panel: Claude drives the browser from a Claude Code session. A new Claude Code session then lists the claude-in-firefox tools; pick a tab with tabs_context and go.

Development

npm run watch        # rebuild dist/ on change
./scripts/check      # prettier + tsc + eslint + vitest + build + clean-code audit (the pre-push gate)
./scripts/e2e        # real headless Firefox through web-ext; screenshots in e2e/screenshots/

Unit tests run the content-script modules under jsdom and the MCP tools against a FakeBridge. The e2e suite starts a local fixture server, launches Firefox headless with the built extension, waits for the native host's socket (isolated via CLAUDE_IN_FIREFOX_SOCKET) and calls every tool through an in-memory MCP client.

Releasing

./scripts/release 0.2.0 --dry-run   # build + lint, publishes nothing
./scripts/release 0.2.0             # bump, sign with AMO, publish xpi + updates.json

scripts/release needs addons.mozilla.org API credentials in config.env (see config.env.example; the file is gitignored). It bumps extension/manifest.json and package.json, builds, signs dist/extension on the unlisted channel, regenerates updates.json (previous versions kept, sha256 hash per file via scripts/build-updates-json.ts), commits and tags v<version>, pushes, and creates the GitHub Release with the .xpi and updates.json attached. The update_url always resolves through releases/latest/download/updates.json, so the newest release is what installed copies see.

Limitations

• Events are synthetic (isTrusted is false). Links, buttons, form controls and framework handlers work; file pickers, popups and anything needing a real user gesture do not. Firefox has no chrome.debugger equivalent for trusted input.

• Extensions cannot run on about:, moz-extension:, addons.mozilla.org or the PDF viewer.

• prefers-color-scheme cannot be emulated per tab; resize_window resizes the real window.

• javascript_tool evaluates in the page's world; pages whose CSP forbids eval fall back to the content-script world (DOM visible, page globals not), and the result says so.

License

MIT