I mostly use chrome these days, for two reasons.
First, at work I use firefox for actual work; I'm a software developer and my company's main product is a web application. Web development is kind of hard on a browser; I'm often clearing my sessions or cache or cookies, which is really inconvenient if I'm actually trying to use the browser at the same time. Having my personal browsing (e.g. enworld) in a completely separate browser insulates me from that. And besides, I get a separate block of active windows in the toolbar.
Second, Chrome can remember the contents of tabs you closed recently, including input form contents you haven't actually posted yet! At least twice this has saved me when I typed in a long update, previewed it, decided it looked good, adn then closed the window because I forgot I was previewing instead of actually posting it. With firefox I would have had to redo the whole thing. With Chrome, the recently closed tabs list kept the whole thing for me.