A fresh new session

gemini mcp add playwright npx -y @playwright/mcp@latest --browser firefox
gemini # Just prompt now and tell it to use the Playwright MCP

An already launched session (with your cookies!)

firefox --marionette
gemini mcp add firefox-devtools npx firefox-devtools-mcp --connect-existing --marionette-port 2828
gemini # Just prompt now and tell it to use the Firefox-Devtools MCP