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.

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

Pass options either as args or env vars. Examples:

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

Option C — Helper script (local dev build)

Try it with MCP Inspector

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

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.

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:

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
• Snapshot/UID: take/resolve/clear (take supports optional saveTo)
• Input: click/hover/fill/drag/upload/form fill <<<<<<< HEAD
• Network: list/get (ID‑first, filters, always‑on capture; both support optional saveTo)
• Console: list/clear (list supports optional saveTo) =======
• Network: list/get (ID‑first, filters, always‑on capture)
• Downloads: list_downloads/clear_downloads (always‑on capture), set_download_behavior (allow/deny/default)
• Console: list/clear

6873767 (Bug 2055949 - [firefox-devtools-mcp] expose download events to firefox devtools mcp)

• 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, 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.

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 with Claude Code's Read tool without impacting context size.

Local development

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):

    root@kitploit:~

    "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):

    root@kitploit:~

    "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.