Add manifest.firefox.json for Firefox MV3 (gecko ID, background scripts instead of service_worker, options_ui). Introduce browser-polyfill.js shim so all modules use whichever API is available (browser.* or chrome.*). Add build.sh to target chrome, firefox, or both. https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw