fix: clarify Firefox experimental support for v0.4.1

[kbuffardi]

Closes #59

Summary

• Show Firefox users exactly this terminal compatibility warning on startup: Firefox support is still experimental. Brave, Chrome, Chromium, or Edge browsers are recommended.
• Keep Firefox build, smoke validation, and signed unlisted-XPI support, but stop generating the unsigned Firefox ZIP in release packaging.
• Bump manifest, package, and lockfile versions to 0.4.1.
• Update release documentation and AMO metadata for the revised distribution flow.

Verification

• npm run lint
• npm run build
• npm run test:e2e (177 passed)
• npm run test:browser:firefox
• npm run version:check
• npm run release:check-version
• npm run package:release (Chrome, Edge, Brave, and Chromium ZIPs only)