freema/firefox-devtools-mcp

npm version CI codecov License: MIT

Model Context Protocol server for automating Firefox via WebDriver BiDi (through Selenium WebDriver). Works with Claude Code, Claude Desktop, Cursor, Cline and other MCP clients.

Repository: https://github.com/mozilla/firefox-devtools-mcp

Note: This MCP server requires a local Firefox browser installation and cannot run on cloud hosting services like glama.ai. Use npx firefox-devtools-mcp@latest to run locally, or use Docker with the provided Dockerfile.

Requirements

• Node.js ≥ 20.19.0
• Firefox 100+ installed (auto‑detected, or pass --firefox-path)

Install and use with Claude Code (npx)

Recommended: use npx so you always run the latest published version from npm.

Option A — Claude Code CLI

claude mcp add firefox-devtools npx firefox-devtools-mcp@latest

Pass options either as args or env vars. Examples:

# Headless + viewport via args
claude mcp add firefox-devtools npx firefox-devtools-mcp@latest -- --headless --viewport 1280x720

# Or via environment variables
claude mcp add firefox-devtools npx firefox-devtools-mcp@latest \
  --env START_URL=https://example.com \
  --env FIREFOX_HEADLESS=true

Option B — Edit Claude Code settings JSON

Add to your Claude Code config file:

• macOS: ~/Library/Application Support/Claude/Code/mcp_settings.json
• Linux: ~/.config/claude/code/mcp_settings.json
• Windows: %APPDATA%\Claude\Code\mcp_settings.json

{
  "mcpServers": {
    "firefox-devtools": {
      "command": "npx",
      "args": ["-y", "firefox-devtools-mcp@latest", "--headless", "--viewport", "1280x720"],
      "env": {
        "START_URL": "about:home"
      }
    }
  }
}

Option C — Helper script (local dev build)

npm run setup
# Choose Claude Code; the script saves JSON to the right path

Try it with MCP Inspector

npx @modelcontextprotocol/inspector npx firefox-devtools-mcp@latest --start-url https://example.com --headless

Then call tools like:

• list_pages, select_page, navigate_page
• take_snapshot then click_by_uid / fill_by_uid
• list_network_requests (always‑on capture), get_network_request
• screenshot_page, list_console_messages

CLI options

You can pass flags or environment variables (names on the right):

• --firefox-path — absolute path to Firefox binary
• --headless — run without UI (FIREFOX_HEADLESS=true)
• --viewport 1280x720 — initial window size
• --profile-path — use a specific Firefox profile
• --firefox-arg — extra Firefox arguments (repeatable)
• --start-url — open this URL on start (START_URL)
• --accept-insecure-certs — ignore TLS errors (ACCEPT_INSECURE_CERTS=true)
• --connect-existing — attach to an already-running Firefox instead of launching a new one (CONNECT_EXISTING=true)
• --marionette-port — Marionette port for connect-existing mode, default 2828 (MARIONETTE_PORT)
• --pref name=value — set Firefox preference at startup via moz:firefoxOptions (repeatable)
• --enable-script — enable the evaluate_script tool, which executes arbitrary JavaScript in the page context (ENABLE_SCRIPT=true)
• --enable-privileged-context — enable privileged context tools: list/select privileged contexts, evaluate privileged scripts, get/set Firefox prefs, and list extensions. Requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1 (ENABLE_PRIVILEGED_CONTEXT=true)

Note on --pref: When Firefox runs in automation, it applies RecommendedPreferences that modify browser behavior for testing. The --pref option allows overriding these defaults when needed.

Connect to existing Firefox

Use --connect-existing to automate your real browsing session — with cookies, logins, and open tabs intact:

# Start Firefox with Marionette enabled
firefox --marionette

# Run the MCP server
npx firefox-devtools-mcp --connect-existing --marionette-port 2828

Or set marionette.enabled to true in about:config (or user.js) to enable Marionette on every launch.

BiDi-dependent features (console events, network events) are not available in connect-existing mode; all other features work normally.

Warning: Do not leave Marionette enabled during normal browsing. It sets navigator.webdriver = true and changes other browser fingerprint signals, which can trigger bot detection on sites protected by Cloudflare, Akamai, etc. Only enable Marionette when you need MCP automation, then restart Firefox normally afterward.

Tool overview

• Pages: list/new/navigate/select/close
• Snapshot/UID: take/resolve/clear
• Input: click/hover/fill/drag/upload/form fill
• Network: list/get (ID‑first, filters, always‑on capture)
• Console: list/clear
• Screenshot: page/by uid (with optional saveTo for CLI environments)
• Script: evaluate_script
• Privileged Context: list/select privileged ("chrome") contexts, evaluate_privileged_script (requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1)
• WebExtension: install_extension, uninstall_extension, list_extensions (list requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1)
• Firefox Management: get_firefox_info, get_firefox_output, restart_firefox, set_firefox_prefs, get_firefox_prefs
• Utilities: accept/dismiss dialog, history back/forward, set viewport

Screenshot optimization for Claude Code

When using screenshots in Claude Code CLI, the base64 image data can consume significant context. Use the saveTo parameter to save screenshots to disk instead:

screenshot_page({ saveTo: "/tmp/page.png" })
screenshot_by_uid({ uid: "abc123", saveTo: "/tmp/element.png" })

The file can then be viewed with Claude Code's Read tool without impacting context size.

Local development

npm install
npm run build

# Run with Inspector against local build
npx @modelcontextprotocol/inspector node dist/index.js --headless --viewport 1280x720

# Or run in dev with hot reload
npm run inspector:dev

See CONTRIBUTING.md for more details on local development, testing, and CI.

Troubleshooting

• Firefox not found: pass --firefox-path "/Applications/Firefox.app/Contents/MacOS/firefox" (macOS) or the correct path on your OS.
• First run is slow: Selenium sets up the BiDi session; subsequent runs are faster.
• Stale UIDs after navigation: take a fresh snapshot (take_snapshot) before using UID tools.
• Windows 10: Error during discovery for MCP server 'firefox-devtools': MCP error -32000: Connection closed

  • Solution 1 Wrap with cmd /c (details):

    "mcpServers": {
      "firefox-devtools": {
        "command": "cmd",
        "args": ["/c", "npx", "-y", "firefox-devtools-mcp@latest"]
      }
    }
    

  • Solution 2 Use the absolute path to npx (adjust extension — .cmd, .bat, .exe, or .ps1 — to match your setup):

    "mcpServers": {
      "firefox-devtools": {
        "command": "C:\\nvm4w\\nodejs\\npx.ps1",
        "args": ["-y", "firefox-devtools-mcp@latest"]
      }
    }
    

Versioning

• Pre‑1.0 API: versions start at 0.x. Use @latest with npx for the newest release.

Contributing

See CONTRIBUTING.md for how to file issues, run tests, and work on the project locally.

Author

Maintained by Mozilla.