Though I use Firefox for the majority of my browsing, I also use Safari and Google Chrome. However, I have a few small complaints about all three browsers.
In my personal history, I Firefox has been the default for a few years already. I haven't been using Internet Explorer on my computer since high school, and Firefox just works for me. On my PC however, I have Chrome and Safari. Both work well (I really like Chrome's drag out feature), but it just doesn't match up to Firefox. I like my extensions, I like my easy searching, and I like how it works. I've also tried Flock for uploading my pictures to FlickR and Photobucket, but the new versions are slower and heavier on the memory. Google Chrome would be my second choice to Firefox. I have to say, I've never tried Opera, so I can't speak for that.