not a new installation:
avast was running for monthes,
thunderbird only for a few weeks without any problem
and it suddenly failed yesterday, from first boot in rhe morning
browser OK but no POP neither SMTP, same for OE and thunerbird:
first connection takes a long time then time-out;
after that one, next immediately reply an error without message
no other antivirus installed
I stopped avast (first time, stop was inoperant, I had to do it twice)
then restarted, it was OK
I then tried to reboot, to stop Avast again, but cannot reproduce