Well, much more important is to have your antivirus up-to-date than to be able to see some animated gifs anyway... waiting few days to update antivirus program, even if new update is already out, is not the cleverest thing to do...
Second of all, I'm hundred percent sure, that has nothing to do with avast!. On my wife's computer, I'm testing another antivirus product, never had avast! installed on that system, and having same problem, but only when I use Mozilla. Sometimes, animated gifs just look frozen. IE never gave me any troubles with animated gifs...
Cheers !
P.S. You better update your avast! virus definitions... best advice.