Well, today I added support for Thunderbird (2.0b2 and greater) for my Growl integration extension. It also works in Firefox for the downloads. You can get it here, but you need to make the sandbox visible in order to see it (it isn’t a public release yet). You should not click the link to go to the extension since it logs you out of amo (another bug that I don’t know the number of, but will be fixed soon). Copy and paste it, and all should work out.
It only says support for Firefox on that page, but that’s only because of Bug 374890. It really does support Firefox 2.0, Thunderbird 2.0b2, and Flock 0.8!
It will display notifications for download start, download finish, download canceled, download failed, and new mail notifications (depending on which application you install it in). If you have any suggestions, please let me know!
EDIT: There seems to be some possible issues for those using 18.104.22.168 of Firefox. Please make sure you have the latest version!
EDIT2: OK, there seems to be some confusion about the url. Sadly, this is in the sandbox, which means there’s a nice complication. You have to log in/create an account at the site, go to your user preferences, and then enable the sandbox. From there, the url will work just fine – otherwise you’ll get an “Addon not found!” error (see Bug 374406 and friends)