As far as cons go, well you cant use firefox for active x applications such as microsoft updates, but to solve that just make sure autoupdate is turned on if your using XP. I guess another con is that some websites are IE specific but they are few and far between. Other than that, which to me, those really arent that bad, there arent any cons to firefox.
Just to add a little to this if I may.
The only con i really see in using firefox, is not being able to use on-line virus scans.
But they have even been able to straighten that out a little.
Housecall Java Online Scan<---For those who use Firefox
Other than that I find it a superior browser to IE and I for one will never go back to that prehistoric creature. Multi tabbed browsing is the way to go.