When importing Thunderbird messages into Entourage (you might want to take a look at this newsgroup posting if you cannot get Entourage to recognize your Thunderbird mailboxes), all messages appear as unread and lose all their other status information (flagged, replied, ...)
This AppleScript sets Entourage's message status flags to reflect what they were set to in Thunderbird (tested on OS 10.4.6 with Entourage 2004)
[robg adds: I haven't tested this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060528130736421