Doron has written a Gmail Notifier extension for Firefox. It adds a button to your toolbar that displays the number of unread emails you have in your Gmail account. Like how I have 0 new emails (I'm a lonely, lonely guy) in the screenshot below.
Neat work, and this is still early stages for Gmail Notifier. Doron has promised Gmail/Mozilla integration and this (Gmail Notifier) seems to be part of that exciting project. Check back with Doron for more later - this should lead to something really neat.