There is no option in the firefox download function to have it scan your downloads, if this worked before then you had an add-on that you were able to enter the location that Tech gave so files would be scanned after they were downloaded.
Firefox 3.0 introduced a new function that it was meant to have your installed AV scan downloads by default, but it still didn't give the user the option to enter the path for the file to scan the download. Unfortunately this simply doesn't work with many AVs and avast is one that it doesn't work with.