I voted Firefox due to my long term use (been using it since 1.5) and reliance on the program to deliver all my web desires right to my lovely little laptop. With my crap laptop (256 ram, 2.0ghz pentium 4 processor, XP, not really my main comp, just for around the home web design) I can happily load it faster than I can load IE.
For FF: The problem lies in that 2.0 was faster (IMO) although 3.0 has improved the security even further. So speed for security was a fine compensation for me (which should be enough for most people).
For IE: IE is native to it's operating system, and being that it's integrated into the system you're just naturally going to use it at some point. You don't need to download anything (unless they update on ya) and you know you can trust the source (I trust Mozilla fully, but from a non-techie stand point, I can understand the loyalty to Microsoft).
I'd go on, but I'll leave it here for someone else to pick up on.
(Web Dev Toolbar+Firebug=Ultimate package for web debugging
)