========================================= Firefox DevTools MCP for AI Agent Testing ========================================= Overview ======== The ``firefox-devtools-mcp`` is a Model Context Protocol (MCP) server that enables AI assistants (such as Claude Code) to automate and interact with Firefox through WebDriver protocols (BiDi and Classic). This tool is particularly useful for: - Automating browser interactions for testing and debugging - Attempting to reproduce issues on live websites - Capturing Firefox debug logs (MOZ_LOG) while controlling the browser - Dynamically changing Firefox configuration without restarting The MCP server provides programmatic access to a Firefox build, including DevTools capabilities such as page navigation, DOM inspection, network monitoring, console logs, user input simulation, and Firefox logging. It isn't complete yet, but can already be useful. This documentation refers to a Mozilla-specific fork with enhanced logging capabilities: https://github.com/padenot/firefox-devtools-mcp Key Features ------------ - **WebDriver BiDi Protocol**: Uses the modern WebDriver BiDi protocol via Selenium WebDriver - **WebDriver Classic Fallback**: Uses WebDriver Classic (Marionette) for features not yet ported to BiDi (notably Chrome context usage) - **Local Firefox Support**: Can connect to custom Firefox builds (e.g., local development builds), or typical release/beta/nightly/esr builds, present on the machine - **Real-time Interaction**: Provides live access to browser state and network activity - **Firefox Output Capture**: stdout/stderr capturing, including setting and reading MOZ_LOG Prerequisites ============= Required Software ----------------- - **Node.js**: Version 20.19.0 or higher - **Firefox**: system installation (all channels supported) or local build - **Claude Code** (or compatible MCP client): For AI-assisted automation Installation and Configuration =============================== Claude Code can install and configure the MCP server with a single command: .. code-block:: bash claude mcp add firefox-devtools npx @padenot/firefox-devtools-mcp This command registers the MCP server with your Claude Code configuration for the current project (typically ``project/.claude/``). If a global installation is preferred (so that it works from multiple project directories), it can be installed at the user scope: .. code-block:: bash claude mcp add firefox-devtools npx @padenot/firefox-devtools-mcp --scope user This changes ``~/.claude.json``, either in the project section, or in the global section (if installed at the user scope). After installation and/or configuration restart Claude Code to load the new MCP server: .. code-block:: bash # Exit Claude Code, then restart it, continuing the last session claude -c The MCP server will automatically start when Claude Code initializes. Configuration options --------------------- The MCP server supports several command-line options. It's preferable to start with no options, but they exist: .. list-table:: :widths: 25 75 :header-rows: 1 * - Option - Description * - ``--firefox-path `` - Path to Firefox executable (system Firefox used if not specified) * - ``--headless`` - Run Firefox without UI (useful for automated testing) * - ``--viewport `` - Set initial viewport size (e.g., ``1280x720``) * - ``--profile-path `` - Use a specific Firefox profile directory * - ``--start-url `` - Initial URL to load (default: ``about:home``) * - ``--accept-insecure-certs`` - Ignore TLS certificate errors (use with caution) * - ``--firefox-arg `` - Additional arguments to pass to Firefox * - ``--env `` - Set environment variables for Firefox (can be used multiple times) * - ``--output-file `` - Path where Firefox output (stdout/stderr) will be captured (optional) Example with multiple options: .. code-block:: json { "type": "stdio", "command": "@padenot/firefox-devtools-mcp", "args": [ "--firefox-path", "/home/developer/firefox/obj-x86_64-pc-linux-gnu/dist/bin/firefox", "--headless", "--viewport", "1920x1080", "--env", "MOZ_LOG=HTMLMediaElement:5" ] } Available Capabilities ====================== The ``firefox-devtools-mcp`` provides tools through the MCP protocol. Tool names are generally self-describing. Simply ask your AI assistant to perform tasks - it will use the appropriate tools. This list may not be complete as the server is under active development. Importantly, you can instruct the agent to run a different Firefox than the one it picked, e.g. your own build, or some specific version, by telling it the path: it isn't necessary to configure it. **Page Management:** ``list_pages``, ``new_page``, ``navigate_page``, ``select_page``, ``close_page`` **DOM Interaction:** ``take_snapshot``, ``resolve_uid_to_selector``, ``click_by_uid``, ``hover_by_uid``, ``fill_by_uid``, ``fill_form_by_uid``, ``drag_by_uid_to_uid``, ``upload_file_by_uid`` **Network & Console:** ``list_network_requests``, ``get_network_request``, ``list_console_messages``, ``clear_console`` **Screenshots:** ``screenshot_page``, ``screenshot_by_uid`` **Dialogs & Navigation:** ``accept_dialog``, ``dismiss_dialog``, ``navigate_history``, ``set_viewport_size`` **Firefox Management:** ``get_firefox_info``, ``get_firefox_output``, ``restart_firefox`` **Script Evaluation:** ``evaluate_script`` (content), ``evaluate_chrome_script`` (privileged) **Chrome Context (privileged code):** ``list_chrome_contexts``, ``select_chrome_context``, ``evaluate_chrome_script`` Working with MOZ_LOG ==================== Firefox's MOZ_LOG system provides detailed debug output from various modules. The ``firefox-devtools-mcp`` captures this output automatically when environment variables are set. You can enable MOZ_LOG with something like this: .. code-block:: text Developer: "Enable HTMLMediaElement logging at level 5" AI uses: restart_firefox(env=["MOZ_LOG=HTMLMediaElement:5"]) Common MOZ_LOG Modules ----------------------- For a useful list of available MOZ_LOG modules and their descriptions, see ``toolkit/content/aboutLogging/aboutLogging.mjs`` in the Firefox repository, or search searchfox.org with something like this `searchfox query `__. Chrome Context Access ===================== For debugging privileged Firefox code (chrome/XUL), enable system access: .. code-block:: text restart_firefox(env=["MOZ_REMOTE_ALLOW_SYSTEM_ACCESS=1"]) Then use ``list_chrome_contexts``, ``select_chrome_context``, and ``evaluate_chrome_script`` to access ``gBrowser``, ``Components``, and other privileged APIs. At the moment, the MCP relies on WebDriver Classic (Marionette) for Chrome context APIs. While WebDriver BiDi and Classic are meant to work seamlessly together, some WebDriver BiDi features might not work when getting executed for chrome browsing contexts. Chrome support for WebDriver BiDi is tracked on the following `meta bug `__.