Only problem I've had, which is REALLY annoying, is it seems to have a significant memory leak. But then I've noticed this in the past, as well. Just particularly bad with the latest version, at least in my experience.
Noticed it one night, about a week ago, while I was kicking around here and a couple other sites. Things started loading slower and slower, and then would hang a bit. Then I tried to open a folder on my computer and it took a good 30 seconds to come up.
Took a look in task manager, and firefox was using 270MB's of memory, even AFTER I'd closed all tabs
Tells ME something's rotten in Denmark..... I guess make your own conclusions.
I've been able to reproduce it over and over again...... Regularly have to close and restart it, to keep things running normally.
Sure, it's free.... But there's very definitely something amiss....
Hopefully they get it squared away soon.