Just dumped Firefox after using it since it was beta. I find it's running slow and skipping video, hangs on gmail or google docs, often chews up well over a gig of RAM, etc. I suspect Google is doing this deliberately to get people to migrate to Chrome, but I did and everything runs much, much faster with fewer resources.
One of the downsides of the Internet is that it allows like-minded people to form communities, and sometimes those communities are stupid.
The tinfoil hat wearers may also point out that Google's browser may collect more of your info to give to Google. I don't know that I'd worry about it, but if that kind of thing pisses you off, you may want to choose something else.
I had a so-so run with Chrome myself.
"Gentle in what you do, Firm in how you do it"
- Buck Brannaman