I find Opera tends to be a bit more forgiving when it comes to rendering bad HTML. Of course, this is by design.
I think this boils down to use whichever you prefer since Opera fans will claim Opera's "the best" and Firefox fans will claim the same. One thing, that doesn't mean much, Opera can do that Firefox can't is come VERY close to passing the Acid2 test (Opera 9, which is still in beta I believe DOES pass it).
So, if you use Opera you've got that bragging right.