In my professional opinion, Firefox is a timebomb. The authors had all the right motives, but they are now finding that it wasn't really incompetence that made all the IE vulnerabilities (well, maybe some

), but rather the complexity of really large projects.
FF grew too fast to ferret out the bugs in an orderly manner. I expect that we'll see a lot more FF bugs over the next year or two.
I've never tried opera, but now that it's free, I think I will.
Also, coming from a business environment, IE is THE standard web browser that almost all software is developed for.