Rated 4 out of 5 stars

Pros: ability to mark/delete/archive messages without having to open them, notification by sound, notification by popup, support for Google Apps email accounts.

Cons: redundant mail checking when 2+ browsing windows are open (which negates the option to "Notify the same new message only once"), the menu could be better organised (I prefer Checker Plus for Gmail's menu, but it's Chrome-only), no control over the notification popup's duration.

Overall, this is Firefox's best Gmail notifier.

This review is for a previous version of the add-on (