Dunno what y'all are talking about. Been using Firefox 4.0.1 since it came out with no problems. Kind'a disappointed they took away the refresh key though. :-\
Do you have 100-150 tabs open in 3 FireFox windows while downloading 10-20 files at a time?
If so, maybe you have a better system than I. What operating system are you running?
Version 3 rarely went down on me. 2/3s of my current tabs were created under that version. I've even closed 50 to no success.
For refresh, I've been hitting F5 with much success. But, I too miss the on-screen button.