ff-mcp

ff-mcp connects a local MCP client to Firefox. It gives the client only the browser access that you approve.

The project has two parts:

• A Firefox Manifest V3 extension controls browser access.
• A Python Native Messaging host serves MCP on 127.0.0.1.

The Firefox extension makes the final access decision. The local MCP server cannot bypass this decision. A client can list tab metadata without page access. The client needs separate grants to read a page, interact with a page, run a script, or take a screenshot.

Features

• List tab metadata.
• Request READ, INTERACT, SCRIPT, or SCREENSHOT access for one tab.
• Grant access once, for one document, for one tab session, or for a host until you remove the rule.
• Read bounded page snapshots and CSS query results.
• Click, type, scroll, and navigate with structured operations.
• Run JavaScript only after a separate SCRIPT grant.
• Build persistent rules with a visual editor.
• Audit access decisions and sensitive operations in Firefox.
• Use a generated bearer token on a loopback-only MCP server.

Requirements

• Firefox 150 or newer.
• Python 3.14.
• uv.

One-time SCRIPT execution needs Firefox 153 or newer. Firefox 153 added userScripts.execute(). Other features support Firefox 150 or newer.

Install

Read the user setup guide for complete instructions.

You can also give this repository to a local coding agent. The agent will read AGENTS.md and guide you through setup. You must still select a Firefox profile and approve the add-on in Firefox.

The main commands are:

uv tool install --force .
ff-mcp profiles --json
ff-mcp setup --profile "/path/to/your/profile" --install-addon

Confirm the installation in Firefox. Then open the ff-mcp toolbar popup and select Start.

For extension development, open about:debugging. Select This Firefox. Select Load Temporary Add-on. Then select extension/manifest.json.

Connect an MCP client

Run this command in a private terminal:

ff-mcp connection --show-token

Do not paste the token into chat, an issue, or a tracked file. Put the token in your MCP client's secret store or environment. The endpoint is available only while the extension is connected.

See the user setup guide for Codex, Claude Code, and generic MCP client examples.

Permission flow

1. Call browser_tabs and select a tab ID.
2. Call browser_request_access with the required capabilities.
3. Approve or reject the request in Firefox.
4. Use the approved browser tool.
5. Revoke the grant in Firefox or with browser_revoke.

READ access is enabled by default for these local addresses:

• localhost
• Subdomains of .localhost
• 127.0.0.0/8
• ::1

This default does not grant INTERACT, SCRIPT, or SCREENSHOT access.

Policy rules

Open the extension options page to use the visual policy editor. Each new rule has one main AND group. The main group contains an OR allow group and a NAND exclusion group.

Add host, URL pattern, regular expression, or scheme conditions. You can nest AND, OR, NAND, and NOR groups. Then select the capabilities that the rule grants.

An empty positive group matches nothing. An empty negated group matches everything. Therefore, a new rule stays inactive until you add a condition to its allow group.

The default localhost rule starts with READ access. You can edit or disable it, but you cannot delete it. A persistent approval is automatically added to the main allow group of a new rule.

Regular expressions have a length limit. They cannot use backreferences, lookarounds, or clear nested quantifiers. This is a safety subset. It is not a complete RE2 implementation.

Development

UV_CACHE_DIR=/tmp/ff-mcp-uv-cache uv sync --locked --group dev
UV_CACHE_DIR=/tmp/ff-mcp-uv-cache uv run ruff check .
UV_CACHE_DIR=/tmp/ff-mcp-uv-cache uv run ruff format --check .
UV_CACHE_DIR=/tmp/ff-mcp-uv-cache uv run pytest -q
node --test tests/background.test.js tests/content.test.js tests/policy.test.js tests/rule-model.test.js

Run the Firefox integration test only when you want to start Firefox with a temporary profile:

FF_MCP_RUN_FIREFOX_TESTS=1 \
  UV_CACHE_DIR=/tmp/ff-mcp-uv-cache \
  uv run pytest -q tests/test_firefox_integration.py

Set FIREFOX_BINARY if Selenium cannot find Firefox. The test can find the standard Linux Snap installation without this variable.

The Firefox extension has no runtime third-party dependencies. It also has no build step.

Releases

Check the four synchronized version sources, or bump all of them with one command:

python3 scripts/version.py check
python3 scripts/version.py bump patch  # also accepts minor, major, or an exact X.Y.Z

A tag in the form vX.Y.Z starts the release workflow. CI checks that pyproject.toml, the Python package, the Firefox manifest, and uv.lock all have the same version. The release additionally requires the tag to match that version.

The workflow sends the extension to Mozilla Add-ons for unlisted signing. It verifies the returned XPI. It then attaches the signed XPI and its SHA-256 file to a GitHub release. These generated files stay untracked. It also publishes updates.json at a stable latest-release URL. Signed versions that contain this update URL use the file for automatic self-distributed updates.

Set these secrets in the GitHub release environment:

• AMO_JWT_ISSUER
• AMO_JWT_SECRET

Create the credentials on the AMO API keys page. Unlisted signing does not add the extension to AMO search results.

Security limits

• The extension requests broad site access because it must support user-approved access to many sites. Its internal capability checks are critical.
• Approved page data and browser activity go to the local native host and MCP client.
• Firefox blocks content scripts on restricted pages such as about: pages and the add-ons store.
• A tab-session grant stays active after navigation in that tab. A document grant does not.
• One bearer token defines one local trust domain. Use separate configurations for clients that do not trust each other.
• A click or input operation can cause page actions. Grant INTERACT access with care.
• SCRIPT gives full page control. A main-world script can read and change page-owned JavaScript state. Grant it only to clients and sites that you trust.

See firefox_mcp_extension_findings.html for the design research.

License

MIT. See LICENSE and THIRD_PARTY.md.