Having read through the first 7 pages of this, I've seen a few good arguments for both sides. I come into this from having been an IE4 support tech when it launched, and I've used both Firefox and IE for quite a while.
I agree that IE is excellent for a corporate platform. The integration and update capability within a large unit is ideal for IT teams to keep current, as compared to allowing Joe 6-pack User to do the updating, which is often spotty at best.
For private users (having no experience with Opera), I would have to say Firefox is the better option. One of the reasons that it has stayed ahead of IE in the tabbed browser and addon arena is due to the fact that it was the original one of the two to start it. As such, it does have the better options there.
The security is also much better for the home user. For someone who doesn't have much experience, it does a pretty good job. For someone with more experience, they can lock down the browser even more.
Finally, with Firefox being open source one could almost say that there is a far larger dev team working on the browser at all times. Again, another plus.
I will say I have both IE and Firefox on my system, and I use Firefox unless I must use IE, or I need to check my webpages as I design them. Ultimately, like any other 'which is best', this argument is entirely up to the individual.