um it's not firefox giving you viruses, it's the stupid places you go and the things you download.
there are more virus scripts written for internet explorer, but the gap is being bridged with ff's popularity.
Just download and use NoScript, an add-on for ff, so you can block all scripts and only run the scripts you won't. Since I DL'ed that I haven't gotten a single virus yet.
Of course you also have to be careful with what you download, because it doesn't matter what browser you're using, if you download malware, ain't nothing going to stop it when you open it up.