That has nothing to do with firefox 5 as the avast update doesn’t use a browser to do its updates, so there is something else in the mix.

What errors are you getting when you try to update (try clicking on the pop-up to see if it has more info) ?

What is your firewall ?
Does it allow avast.setup internet access ?

  • If it does delete the entry for it and do a manual update, this should force the firewall to ask permission again.