Good old flame wars. Here's my data point:
I usually run Firefox, but for a few days a while ago suprnova.org (RIP) wouldn't load on the newest Firefox release, so I booted up IE to go to the site. After roughly 30 secs my AV told me I had picked up a trojan, and I had to permanently quarantine three files. I have SP2 and all the latest updates, I run Adaware and Spybot on a regular basis, and I'm on a college network so I never turn off Symantec Corporate Edition or Zone Alarm. And despite all that, getting IE to run half well is a pain in the ass that I just don't want to deal with. Every time I sit down at someone else's computer and see how many vulnerabilities their install of IE has, I'm totally freaked out. Of course, I could explain just what they need to do to keep IE reasonably safe, but for most common users it's so much easier to just install Firefox and learn how to use it.
That being said, I do agree with one point from the anti-Firefox side. If Firefox ever does capture true market share there will be a greater incentive for people to exploit it's vulnerabilities, and it'll certainly become a lot less safe than it is now. That's why I'm pro-Firefox and against spreading it.