You can now receive a Growl notification when a download starts, finishes, fails, or is canceled in Firefox. Clicking on the notification will open the download manager.
In Thunderbird, it displays a notification for new mail (displays the sender and subject). Clicking on the notification will focus Thunderbird.
If you have Chatzilla installed, several notifications can also be displayed! Clicking on a notification will focus Chatzilla.
You can turn off these notifications by changing what notifications Growl displays in the Growl prefpane.
Once you install and restart your browser, you should get a Growl notification indicating that the application has registered.
If you want some other extension to have Growl Notifications support, contact them and point them to this page: http://trac.growl.info/wiki/MozillaGrowlNotifications