I've recently switched from POP to Gmail's IMAP. Whenever I receive a mail with extended characters (e.g. diacritics), they appear as ??? in Thunderbird.
I can confirm the behavior, but viewing the message source I think it's gmails fault. Sending "testing elagané åäö!" I receive (in the message source) testing elagan? ???!
Diacritics look fine inside Gmail's web interface and in the iPhone's mail app.
Reported it to google, evidently it's a know issue. "We are aware of this problem, and our engineers are working diligently to find a solution. We apologize for any inconvenience this issue may have caused." ->INVALID
Seems gmail has fixed this now.