Firefox vs. IE is a religious war, so I won't get into that.
With Gaim you can talk with AIM/MSN/etc. users but it's open source and doesn't have any ads. That was the main thing that made me switch, the ads. AIM kept having ads and sometimes even placed tracking cookies on my computer. The thing that finally tipped me over the edge was when it started automatically changing buddy icons to say "Bigger is Better" (about AOL mail or something) which made me think of certain spam e-mails I get... I switched permanently at that point.