Steven J. Vaughan-Nichols has a report that Firefox 9 was prone to crashes. I did notice Firefox crashing two or three times recently. 9.01 apparently fixes the problem (or more accurately removes a bug fix which caused the unexpected problem). I have 9.01 now on Fedora and haven't noticed any more crashes.
9.01 is also claimed to show noticeable speed improvements. I can't say I have noticed. When I do notice the internet being slow, it's usually because of some third-party content that won't load. Anything on Wordpress takes forever to load.