Well, not for nothing but small business owners in America alone would save $198 million a year if everyone just used IE. Source: NIPOMA Institute
seriously though... if FF dropped off the face of the earth (al la Netscape) the only people who'd care would be a handful of brooding middle-aged programmers, mac posers who like the mac logo more than the OS or safari and, i guess, a couple people in this thread. haha.
Use FF if you don't mind upgrading it every day or two, and don't care if the websites you're looking at don't display/work how they were designed to, and of course if you hate small business owners. hah. Use IE otherwise.