I use Thunderbird on all platforms, but mostly on Macs. Lately I've been trying to streamline my email workflow (who hasn't been?) and thought I'd give Growl a try. Growl came with my installation of Adium, and I'm kinda digging it. I assume you already have Thunderbird 2+ and Growl installed. I cribbed from a post on Neil's World for this post. Unfortunately, the Growl that ships with Adium doesn't seem to have the Extras folder, so you'll still need to get the installer package from their website. You can just open the DMG and copy its Extras folder into your home directory or something.
But! First thing you need is for something to have Thunderbird tell Growl that you have new mail. yamb (Yet Another Mail Biff) seems to be the way to go there. Installing it is as simple as downloading the XPI, then telling Thunderbird about it. In configuration settings, I disabled everything but IMAP (I don't use pop3 or local folders or anything like that), and told it just to notify on my inbox - if it hits one of my mailing list folders I don't care about it.
After that, go ahead and install the GrowlNotify stuff. Neil's world tells you to use "su -" and then run the installer script; I say to "sudo sh ./install.sh" instead. It will stuff things into /usr/local/bin. I grabbed the growlNotify.pl script that was listed in the comments in the Neil's World post, and stashed it into one of my bin dirs that's in my path, although having it in your path isn't necessary as yamb will take a full pathname. I configured yamb to use it as the external notifier executable. You will have to chmod +x it for it to work properly.
I didn't bother modifying the checking interval, as my mailserver actually tells Thunderbird when I have new email. After all that, it Just Worked, and now when I get new email I can see the subject line so I know whether or not it's worth switching to my "mail" virtual desktop.
(Edit: fixed a typo)