Whilst firefox 3 has a function to have downloads scanned, it has a proviso that it doesn't work with all AVs and it doesn't work with avast, so by default your downloads aren't scanned via firefox calling the AV.
If you are having to close the Simple User Interface window, then you have selected the wrong component of avast to scan the download as Alan mentioned in Reply #4 above.
So I would check any download management you are using within firefox (add-on, etc.) and change it to the path given by Alan.
I don't know why the about avast window would pop-up, you say you have registered, but that is the first step, you need to enter the key that was sent to you by email into that License Key... field,
Insert Registration Key - Video.