Rated 3 out of 5 stars

Great idea, needs quite a bit of work.
As mentioned before:
1. It really needs a popup notification (like the original Chrome extension)
2. same for sound notifications
3. Popup mail control doesn't really seem to work, it seems.

Please go on working on this, though, as it's potentially far better than other GMail notifier extensions I've seen for Firefox.

This review is for a previous version of the add-on (0.1.5.1-signed).