Hmmm... will this thread get closed because of religious discussion? I mean, this is, after all, a holy war...
I've used both. I generally prefer FF to IE for a couple of reasons.
1. Most people developing nastiness (viruses and the like) are developing them to attack IE, and not FF. So by using FF, I'm using something they're not trying to attack. That's just safer, period.
2. I've not noticed a speed difference between them. The only thing I've noticed are a few graphics differences in how they display info. They display it at the end, but FF gives me a little more flicker (for example, in the "Hop To" box at the bottom of this page).
3. I've used other open-source initiatives, and while they can suffer from bloatware (the addition of thousands of useful and useless things), it's not a major concern with FF. Yet... [img]smile.gif[/img]
That being said, there are a couple of things I can't access with FF. Some sites require your browser to be at least IE 5.5, and based on the checks they do, FF doesn't match up. You can argue for a poorly designed site (and probably win), but I'd also look to the browser for an easy way to pretend to be IE 5.5 and higher.