Moxie girl may have had the same problem I had and just now solved it. I've been searching for a solution since last night here and Thunderbird's site. Nothing mentioned what I did, so this, I hope will help:
I did EVERYTHING that I read. Nothing worked. Thunderbird would either not see the proxy or the AV wasn't working. Finally, I wiped out all of Thunderbird. If using XP, make SURE and wipe out the folder under C:\Documents and Settings\{your account folder}\Application Data\Thnderbird. Then, run the Avast wizard and choose manual setup, unselect all, then keep hitting next until your done.
Now, make sure Avast is up and running and reinstall Thunderbird. USE ONLY these settings!:
POP3 Server Name - 127.0.0.1 PERIOD! No # or : or localhost:, no anything!
Username - {your user name} PERIOD! DITTO
SMTP Server Name = 127.0.0.1 PERIOD! DITTO
Finish the install. Then, if all is well, it should work.
Good luck!