I've been running Firefox for the last several months. Last I looked it was version 4, but I ran OS updates to day and it looks like I'm now running version 5. No noticeable difference that I can tell, Firefox has always been very fast since I started using it. I'm running it in a 64 bit Linux OS with 8Gb of RAM and 3GHz quad core CPU off of a SSD, so I wouldn't expect to see much difference.