I just loaded Firefox 4.0 and I have this exact same problem. Sometimes I never can get Firefox to stay. I am using Windows XP 32 bit. My computer is the AMD Phenom II 1090T 6 core Thuban CPU with a Gigabyte AMD GA-890GPA-UD3H 890GX chipset motherboard, 4GB of DDR3 1333 ram, Western Digital Caviar Black WD1002FAEX 1TB 6 GB/second hard drive and PowerColor AX6850 1GBD5-DH Radeon HD 6850 video card. The previous version worked fine. My wife can no longer do her online school work with this computer now.

Like wbliss I too have an AMD Phenom II X6 1090T CPU. Similarly I also have a Gigabyte AMD 880 chipset motherboard and 8GB of DDR3 1600 RAM and Radeon HD 5770 video card.

I had to downgrade to version 3.6.16 in order to get Firefox functioning properly again. Apparently my wife's professor was aware that there is a problem with Firefox 4.0. They warned all the online college students not to install Firefox 4.0. I was just not aware of that warning. So this is not an isolated incident on just a couple computers. I hope there is a fix for Firefox 4.0 soon.

I was able to fix the problem by going to the Tools->Options menu item, going to the Advanced panel and in the General tab deselecting the 'Use hardware acceleration when available' check box.

I am guessing this may be a compatibility issue with the ATI Radeon HD driver and Firefox 4. For now I am happy with the new Firefox and don't notice any slow down from disabling hardware acceleration.

I have a Radeon HD 5750, disabling 'Use hardware acceleration when available' solved the issue for me too.

Added 11.Mai: Firefox 4 still crashes with hardware acceleration enabled and Catalyst 11.5 installed.

I still experience this problem with Firefox 5 and now too with the new Thunderbird 5.

To fix the problem in Thunderbird I had to go to Tools->Options menu, Advanced tab, and click the Config Editor... button. Locate layers.acceleration.disabled in the preference name list and change the value to true.

I just upgraded to the new Firefox 7 and still have this problem if I enable hardware acceleration. Also still have this problem with Thunderbird 7.

Only 32 bit Windows version of Firefox 4.0 or higher have problems with x6 core processors. Linux and Firefox x64-bit version for Windows work without crashes. I cannot understand why Mozilla dont fix this annoying problem.

Edit: I found another way to solve the problem if you dont want change any settings in Firefox or Thunderbird. Rightclick on the desktopicon for the program you want to change and go to Properties then Compatibility and choose Windows 2000 (XP compatibelity does not work) see pictures below