No problems with FF4 (on Linux Mint 10) yet. I do wish the very handy zoom function came with a ‘reset to normal size’ button, though. ‘Control zero’ does the job, but a button would be better.
I had a lot of trouble with FF3.6 crashing and eventually went back to FF3.1. It turned out the real problem was a bad RAM address that 3.6 accessed but 3.1 didn’t, as I discovered when I upgraded my RAM. People having problems with frequent crashes of any FF edition might want to think about new RAM.
Do you know of a way to get the reload button back to the left side on the Linux version? Drives me nuts.