I like it, use it for my school account. Though with all the customization available to Thunderbird and Firefox, I still can't find out how to get the darn attachment bar to appear above the message instead of at the bottom of the window - or even if you can do this at all. It's not too bad really.
Let's hope it doesn't... a nice proggie it is. Eventually, it might become a standard as companies shall realize it is much more secure than outlook! Ehehe... well, that's only what I hope.