zen-devtools-mcp

zen-devtools-mcp

Wraps @mozilla/firefox-devtools-mcp so it attaches to your already-running Firefox-based browser — Zen Browser, Firefox, or any fork — instead of launching a throwaway instance.

The upstream --connectExisting mode requires the browser to have been launched with Marionette enabled. Plain relaunches (clicking the dock icon, open -a Zen) don't set that, so attaching fails. This wrapper fixes that: if Marionette isn't up, it gracefully quits the browser (session restore keeps your tabs), relaunches it with --marionette, waits for the port, then starts the MCP.

Install / run

No install needed — run it straight from GitHub:

npx github:sushruth/zen-devtools-mcp
bunx github:sushruth/zen-devtools-mcp

Options

Flag	Default	What it controls
--app	Zen	macOS app name for open -a / quit
--process	zen	process name for pgrep -x
--port	2828	Marionette port to probe and listen on

Examples:

# Firefox instead of Zen
npx github:sushruth/zen-devtools-mcp --app Firefox --process firefox

# Zen with a custom Marionette port
npx github:sushruth/zen-devtools-mcp --port 2829

MCP client config

OpenCode example:

"zen-devtools": {
  "type": "local",
  "command": ["npx", "-y", "github:sushruth/zen-devtools-mcp"],
  "enabled": true
},
"firefox-devtools": {
  "type": "local",
  "command": ["npx", "-y", "github:sushruth/zen-devtools-mcp",
              "--app", "Firefox", "--process", "firefox", "--port", "2829"],
  "enabled": true
}

Claude Code (claude mcp add-json zen-devtools '<json>' or .mcp.json):

"zen-devtools": {
  "command": "npx",
  "args": ["-y", "github:sushruth/zen-devtools-mcp"]
}

Notes

• Session restore preserves tabs across wrapper-triggered restarts.
• Marionette can't be enabled on a live browser process — a restart is unavoidable the first time.
• macOS-only (open, osascript, pgrep).
• Passes --connectExisting --enableScript to the underlying MCP; it inherits stdio from its parent.