To toss my $.02 worth in, I too use FF for about 95% of my browsing. There are a couple of sites needed for work that REQUIRE use of IE. (I won't even go there! ) For development, I test in IE and FF, using IE as the predominate one, since that's what is the predominate browser out there. And yes, there is a noticable difference between the two. But, like I said, since IE is the main one, that's the one I focus on.
Just a quick question...does anyone know how much of a difference there is between Firefox and Netscape? Is it enough to warrant installing just for test purposes?