The problem with using the built in Firefox downloader is that you are given absolutely no indication the download has even started, the progress, the speed and even how big the file you're downloading actually is. No quick change of the download target folder, no obvious way to pause, continue if disconnected, cancel or delete it either. It is one of the least satisfactory elements of Firefox IMHO and very good reasons for using a purpose designed FireFox download manager.
That is why I use the DownLoadThemAll add-on. This is excellent because it covers all those stated requirements and more. However, unfortunately, some (free) download web site just don't like it. You'll get a FireFox document error message instead of the download or a warning you've exceeded your download limit even before you've started.
Avast though has never given me any problems at all with it.
Because of that occasional DownLoadThemAll issue a long time ago now I also installed Opera. Its built in downloader is far better designed than the FireFox one although not as feature packed as DownLoadThemAll. It has never let me down when the the latter has had problems and has also never caused any conflict with Avast either.