Firefox DevTools MCP

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 @mozilla/firefox-devtools-mcp@latest to run locally, or use Docker with the provided Dockerfile.

Security

Browser MCP servers carry inherent risks. A few key practices:

• Use a dedicated Firefox profile. Never run the server against your regular profile — the agent has access to whatever the browser can reach, including cookies and saved sessions.

• Be cautious about which sites you visit. Pages can return content designed to manipulate the agent (prompt injection). Stick to sites you control or trust.

• Enable only the tool modules you need. Higher presets such as --tool-preset developer (script, debugging) and --tool-preset mozilla (privileged context) significantly expand what the agent can do.

See SECURITY.md for a full breakdown of risks and how to report vulnerabilities.

Related MCP server: LCBro

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 @mozilla/firefox-devtools-mcp@latest

Pass options either as args or env vars. Examples:

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

# Or via environment variables
claude mcp add firefox-devtools npx @mozilla/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", "@mozilla/firefox-devtools-mcp@latest", "--headless", "--viewport", "1280x720"],
      "env": {
        "START_URL": "about:blank"
      }
    }
  }
}

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 @mozilla/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

• list_downloads (always‑on capture), set_download_behavior

• 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)

• --tool-preset — select which tool modules to enable: slim, basic (default), developer, mozilla, or all. See Tool modules and presets. (TOOL_PRESET)

• --tools — explicit list of tool modules to enable, overriding --tool-preset entirely (e.g. --tools pages network script). See Tool modules and presets.

• --enable-script — deprecated, use --tool-preset developer or --tools ... script debugging. Selects the developer tool preset. (ENABLE_SCRIPT=true)

• --enable-privileged-context — deprecated, use --tool-preset mozilla or --tools ... privileged prefs. Selects the mozilla tool preset. Requires MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1 (ENABLE_PRIVILEGED_CONTEXT=true)

• --android-device — enable Firefox for Android mode; value is the ADB device serial (e.g. emulator-5554). Run adb devices to list connected devices. Omit the value or use auto to select the single connected device automatically.

• --android-package — Android app package name, default org.mozilla.firefox. Other packages: org.mozilla.firefox_beta for Firefox Beta, org.mozilla.fenix for Firefox Nightly, org.mozilla.fenix.debug for Firefox Nightly Debug, org.mozilla.geckoview_example for geckoview (ANDROID_PACKAGE)

• --log-file — write MCP server logs to a file instead of stderr. Useful for debugging sessions with MCP clients that hide server output. Set DEBUG=* to also include verbose debug logs. Example: --log-file /tmp/firefox-mcp.log

Tool modules and presets

Tools are grouped into modules. You choose which modules to expose either with a named preset (--tool-preset) or with an explicit list (--tools). When both are given, --tools wins and the preset is ignored.

Modules: pages, snapshot, input, network, console, screenshot, utilities, management, webextension, profiler, screencast, script, debugging, prefs, privileged.

Presets (each is a superset of the previous):

• slim — pages, snapshot, input, network, console

• basic (default) — slim plus screenshot, utilities, management, webextension, profiler, screencast

• developer — basic plus script, debugging

• mozilla — developer plus prefs, privileged

• all — every module

# Use the developer preset (adds script and debugging tools)
npx @mozilla/firefox-devtools-mcp --tool-preset developer

# Enable only the modules you need
npx @mozilla/firefox-devtools-mcp --tools pages network console

The prefs and privileged modules require MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1 and are only available in the Mozilla-internal build; the public package silently skips them even if requested.

Useful preferences (--pref)

• remote.prefs.recommended=false. When Firefox runs in automation, it applies RecommendedPreferences that modify browser behavior for testing. Set remote.prefs.recommended to false to skip those and have a configuration closer to a regular Firefox instance.

• remote.log.level=Trace. Enable verbose WebDriver protocol logs in Firefox. The MCP server will automatically pass the matching log level to geckodriver so both sides log at the same verbosity.

• app.update.disabledForTesting=false. Allow Firefox to automatically download and apply updates. Note that updates may interrupt your session. Requires also setting remote.prefs.recommended=false.

Firefox for Android

Use --android-device to automate Firefox running on an Android device. Requires adb on your PATH and geckodriver, which is managed automatically.

# List connected devices
adb devices

# Launch Firefox for Android on the single connected device
npx @mozilla/firefox-devtools-mcp --android-device auto

# Target a specific device
npx @mozilla/firefox-devtools-mcp --android-device <serial>

# Use Firefox Nightly instead
npx @mozilla/firefox-devtools-mcp --android-device <serial> --android-package org.mozilla.fenix

Port forwarding between the host and device is handled automatically by geckodriver.

Connect to existing Firefox

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

# Start Firefox with Marionette and the Remote Agent (BiDi)
firefox --marionette --remote-debugging-port

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

Both flags are required because the MCP uses both WebDriver Classic (--marionette) and WebDriver BiDi (--remote-debugging-port). If Firefox is only started with --marionette, the MCP server fails to connect and asks you to restart Firefox with both flags.

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/get_page_text (get_page_text supports optional saveTo)

• Snapshot/UID: take/resolve/clear (take supports optional saveTo)

• Input: click/hover/fill/drag/upload/form fill

• Network: list/get (ID‑first, filters, always‑on capture; both support optional saveTo)

• Downloads: list_downloads/clear_downloads (always‑on capture), set_download_behavior (allow/deny/default)

• Console: list/clear (list supports optional saveTo)

• Screenshot: page/by uid (with optional saveTo for CLI environments)

• Script: evaluate_script (with optional saveTo for bulky results)

• 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

• Profiler: profiler_is_active, profiler_start (preset or explicit config), profiler_stop (saves profile to downloads directory)

• Screencast: screencast_start (records the page viewport to a video file in the downloads directory), screencast_stop (requires Firefox 154+)

• Utilities: accept/dismiss dialog, history back/forward, set viewport

Saving bulky output to disk

Large tool output can consume significant context in CLI clients like Claude Code. The screenshot_page, screenshot_by_uid, take_snapshot, list_console_messages, list_network_requests, get_network_request, get_page_text, evaluate_script, and evaluate_privileged_script tools accept an optional saveTo parameter that writes the result to a file instead of returning it inline. saveTo takes one of three forms:

• a file path (relative to the current working directory, or absolute within ~/.firefox-devtools-mcp; parent directories are created)

• an existing directory (a timestamped file is generated inside it)

• true (a timestamped file is generated under ~/.firefox-devtools-mcp/output/)

The response returns the path and byte size. The saved file always holds the full, untruncated data: the inline size safeguards (console message caps, network header truncation, snapshot line caps) never apply to it.

The text-producing tools (everything except the screenshots) also accept preview, a number of characters of the saved output to echo back inline as a short excerpt. Screenshots have no preview.

screenshot_page({ saveTo: "page.png" })
take_snapshot({ saveTo: true })
list_network_requests({ urlContains: "api", saveTo: "network.json" })
evaluate_script({ function: "() => performance.getEntries()", saveTo: true, preview: 2000 })

By default, save paths are restricted: relative paths resolve against the current working directory, and absolute paths are only allowed within ~/.firefox-devtools-mcp. Paths that escape these locations are rejected. Start the server with --unrestricted-save-paths to write to arbitrary locations, including absolute paths outside that directory.

Saved files can then be viewed for instance 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", "@mozilla/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", "@mozilla/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.

License

Licensed under either of MIT or Apache 2.0 at your option.