That's a fair question, in practical terms I don't think it's doing much. I'm worried we'll end up in a state where only one browser engine is able to access the web, so google will have absolute control over web standards like microsoft did in the IE6/7 days. That's kind of already the case and seems inevitable, but using firefox feels like a small thing I can do to register my dislike for a browser monoculture.

Also what others said about the better addons (adblockers and tree style tabs). Firefox may be more annoying to get into a good state but at least it's still possible.