I have noticed that when using Firefox, some websites don't come up. In example, when I worked at the school district, I was the administrator for Spy Sweeper Enterprise, but whenever I wanted to go into the Administration Console, I had to go to another computer that had Internet Explorer on it in order to gain access to the Admin Console without going to the server itself. I never tried using Opera in Firefox's place, but doubt that would work either. Webroot told me over the phone that they don't support Firefox or Opera.
I also noticed, like Kage says, Firefox runs a little slower. I especially notice this on the Cyberguys site. There are also some issues rendering in my opinion. Long story short, I agree with all of you about Firefox. I still use Internet Explorer 6.0 on my workstation, so I use Firefox because I heard it was safer than IE6. But I anticipate upgrading to IE 7 once the official release comes out. Not fond of beta testing on my main computer.