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 has two halves: the browser-automation tools, exposed to Claude Code as mcp__claude-in-firefox__*, and a sidebar chat that runs a local Claude Code session for you (see Sidebar chat).

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: Kamurafox

Sidebar chat

Click the toolbar icon → Open chat (or press Alt+Shift+C) and a chat opens in the Firefox sidebar, like Claude in Chrome's side panel: ask about the page you are on and Claude answers, reads it, clicks, types and opens tabs through the same browser tools. Each message carries the tab you are looking at, so Claude starts on the right page without being told.

Under the hood the native host runs your own Claude Code for each conversation (claude -p --output-format stream-json --input-format stream-json, resumed with --resume when the process is gone), so it uses your Claude Code login and plan, no API key, and nothing goes through any server of this project. Replies stream as they arrive; tool calls show as collapsible lines with their result, and screenshots Claude takes appear inline (kept in memory only, so they are gone after a restart; one over ~700 KB is replaced by a note, since native messaging frames are capped at 1 MiB).

• Permissions. The claude-in-firefox tools are pre-approved. Anything else Claude Code would ask about (Bash, file writes, other tools) appears in the sidebar as an Allow / Deny card; nothing runs until you answer. Your Claude Code settings and hooks apply as usual. Only this project's MCP server is loaded (--strict-mcp-config), so other MCP servers you registered in Claude Code are not available in the sidebar.

• Model. The picker sends --model (Sonnet, Opus, Haiku, or your default). Changing it mid-conversation restarts the process with --resume, so the history is kept.

• History. The last 20 conversations stay in the extension's storage and can be reopened from the picker; after a browser restart the next message resumes the Claude Code session. New chat starts a fresh one; Stop interrupts the current turn.

• Where it runs. Sessions live in ~/.local/state/claude-in-firefox/chat (Claude Code's working directory, so no project's CLAUDE.md leaks in; claude --resume works from there). The host finds claude on PATH, then in ~/.local/bin and ~/.claude/local; set CLAUDE_IN_FIREFOX_CLAUDE_BIN if yours lives elsewhere (Firefox started from a desktop launcher may have a shorter PATH than your terminal).

• From Claude Code. The bridge also accepts chat.send and chat.transcript, so a script or test can post a turn into the sidebar and read what happened (the e2e suite does this with a fake claude).

From the Claude desktop app

Claude Code sessions (CLI and the desktop app's Code tab) see the tools as soon as scripts/setup has registered the server. The desktop app's Chat tab (claude.ai) does not read Claude Code's MCP servers; it has its own list in claude_desktop_config.json (~/.config/Claude/ on Linux). Add the server there with absolute paths, since the app launches MCP servers with a minimal environment and no PATH:

{
  "mcpServers": {
    "claude-in-firefox": {
      "command": "/absolute/path/to/node",
      "args": ["/absolute/path/to/claude-in-firefox/dist/mcp-server.js"]
    }
  }
}

Restart the app; a Chat conversation then lists claude-in-firefox among its tools and can drive Firefox the way Claude in Chrome drives Chrome. A Chat conversation has no "current tab", so name the tab you mean or let it open one with tabs_create.

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)       /run/user/<uid>/claude-in-firefox.sock   (spawned by Firefox,
        ▲                                                                  one per extension)
        │ MCP over stdio (sidebar chat)                      claude -p ◀────┤ stdin/stdout stream-json,
        └──────────────────────────────────────────────────  per conversation │ one process per chat
                                                                              │ native messaging
                                                                              │ (4-byte length + JSON)
                                                                              ▼
                                                                  extension background.js
                                                                  tabs · screenshots · console/network stores
                                                                  chat store ──▶ sidebar.html (runtime Port)
                                                                              │ 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. For the sidebar it also spawns one claude process per conversation; that process starts its own mcp-server.js, which connects back to the same socket. Bridge wire types live in extension/src/shared/protocol.ts, chat types in extension/src/shared/chat-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, Reconnect, Open chat). A new Claude Code session then lists the claude-in-firefox tools; pick a tab with tabs_context and go, or open the sidebar chat and type.

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 native host's claude runner is tested against tests/fakes/fake-claude.mjs, a stand-in that speaks the stream-json subset the host relies on. 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), calls every tool through an in-memory MCP client, and sends a sidebar-chat turn through the fake claude (CLAUDE_IN_FIREFOX_CLAUDE_BIN), screenshotting the sidebar page.

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.

• The sidebar chat needs Claude Code installed and logged in on this machine; it shows Claude Code's own error when it is not. Stopping a turn asks Claude Code to interrupt and kills the process after 5 s if it does not; the conversation resumes on the next message.

License

MIT