gecko-mcp

An MCP (Model Context Protocol) server that lets AI assistants — Claude Code, Claude Desktop, Cursor, and any MCP client — read pages, take screenshots and manage tabs in Floorp and other Firefox-based browsers (LibreWolf, Waterfox, Zen, Mullvad, Firefox…), using your real, logged-in session.

Think "Claude in Chrome", but for the whole Firefox/Gecko family.

Get started in ~30 seconds:

npx gecko-mcp setup

Registers gecko-mcp with Claude Code, Claude Desktop, Cursor, VS Code (Copilot), Windsurf, Zed, Codex and more — per-project or global. See Setup for manual config and Requirements for the one-time browser step.

Cautious about installing this? Good — you should be. It's small (2 deps, all in src/), the OS keyboard/mouse is locked by default (browser-only until you opt in), releases ship with npm provenance (verifiable against this source), and the full threat model is in SECURITY.md. Read it before you run npx gecko-mcp.

How it works

gecko-mcp talks to the browser through one of two backends, picked automatically:

• Floorp ships a built-in automation HTTP API. Set floorp.mcp.enabled = true in about:config and gecko-mcp uses the fast http://127.0.0.1:58261 API — no extension, richest feature set.
• Any other Gecko browser — launch it with Marionette (the automation engine built into every Firefox fork) and gecko-mcp drives your live session over it. Same tools, same real session.

  Claude Code / Desktop / Cursor
            │  MCP (stdio)
            ▼
      gecko-mcp ──► Floorp :58261 (built-in API)        ─┐
   (this project) ──► Marionette :2828 (any Gecko fork)  ─┴─► your real tabs

Requirements

• A Firefox-based browser installed and running, with automation enabled:
  • Floorp: set floorp.mcp.enabled to true in about:config, restart Floorp.
  • Other forks (LibreWolf / Waterfox / Zen / Mullvad / Firefox): launch the browser with -marionette (see Browser support).
• Node.js ≥ 18.

Setup

Quick start — the setup wizard

npx gecko-mcp setup

An interactive wizard registers gecko-mcp with the AI coding tool(s) of your choice — Claude Code, Cursor, Windsurf, VS Code (Copilot), Gemini CLI, Codex, Zed, Cline (and a copy-paste snippet for Kimi Code, Antigravity, or any other MCP client) — and lets you install it for the current project or globally (all repos). It merges into existing config (and backs it up first).

Non-interactive / scriptable:

npx gecko-mcp setup --list                          # show supported tools
npx gecko-mcp setup --tool claude-code,cursor --scope global
npx gecko-mcp setup --tool codex --scope global --print   # dry run

Manual

Any MCP client works with this server block (no clone/build needed — npx fetches it):

{
  "mcpServers": {
    "gecko": {
      "command": "npx",
      "args": ["-y", "gecko-mcp"]
    }
  }
}

Or with Claude Code's CLI: claude mcp add gecko -s user -- npx -y gecko-mcp.

One-time Floorp step: set floorp.mcp.enabled = true in about:config and restart Floorp so its automation API is available.

Browser support

gecko-mcp picks its backend automatically: if Floorp's :58261 API is reachable it uses that; otherwise it connects to Marionette, the automation engine built into every Gecko browser. To use a non-Floorp browser, launch it once with Marionette enabled:

Marionette listens on TCP 2828 by default. To use another port, set the marionette.port pref in the profile (e.g. via user.js) and start gecko-mcp with a matching MARIONETTE_PORT. Force a backend with GECKO_MCP_BACKEND=marionette.

Note: Marionette must be enabled at launch to attach to your live session. On the Marionette backend, Floorp-only extras (snapshot fingerprints, list_workspaces/switch_workspace, accessibility tree) return a clear "not supported" message — use find / read_page instead. Everything else (tabs, navigation, click, type, forms, screenshots, cookies, real OS input…) works.

Tools

Tabs & reading

Interaction

Most tools target the active tab by default; pass a browserId (from list_tabs) to target a specific tab.

OS keyboard & mouse — locked by default 🔒

The tools below can affect things outside the browser, so they are disabled until you turn them on. With nothing set, gecko-mcp does browser automation only. Unlock them per-session by just asking ("enable OS input", which calls the enable_os_input tool), or persistently with GECKO_MCP_ENABLE_OS_INPUT=1. Lock again with disable_os_input. While locked, these tools refuse with a clear message.

The evaluate tool (run arbitrary page JavaScript) is locked the same way — unlock with enable_evaluate or GECKO_MCP_ENABLE_EVALUATE=1.

Real OS keyboard (Windows) — for React/rich editors and bot-guarded submits that ignore synthetic input:

These produce input a page can't distinguish from a human's, so they drive React/Slate editors and submit composers that synthetic clicks/typing can't. Workflow: click the field to focus it → real_clear / real_type / real_key "Enter".

Safety guard: OS keystrokes go to the foreground window, so before sending anything these tools bring Floorp to the foreground and verify it — if Floorp isn't running or can't be focused, they abort without typing a single key, so input can never leak into another app.

Real OS mouse (Windows) — genuine isTrusted clicks at screen coordinates:

Double guard: the click is sent only when Floorp is verified foreground and the point lies inside Floorp's window rect — a stray coordinate is refused, so a click can never land in another app/window. Coordinates are screen pixels (note display scaling/DPI when mapping from a screenshot).

More interaction & queries