>>how good a product Firefox is<<
FF is NOT a "good" product. It may be better than IE and I'm forced to use it due to the incompatability of sites with Opera, but I don't like it. It has serious memory leak and stability issues that the developers absolutely refuse to fix.
It really does. I think IE7 Beta 3 is already a better product, despite the incompatibilities. I use Safari more often now and I've tried Opera, but I'm not 100% happy with any of them.