There is a tutorial HERE
which will explain things better than I can, but yes - IMO it is as good as anything else out there and does everything you would expect it to. It has realtime scanning of everything important and gives clear recommendations when it finds something. I have used several different antivirus but have not found a reason to change so far.
Site Advisor works with both FireFox and IE and certainly with Avast. The only scan I bother with these days is MS Windows Live ONECARE
. You do need to use IE but a quick scan for baddies takes only a few minutes and as it is made by MS for MS, I am quite happy to run with it.
As far as I am concerned, Avast is as good a freebie as you will get.