Sorry you are having issues. I am running Win 7 Pro, Firefox 4.0.

I had to dis able Extensions: Flashblock and WOT as they were up to date but still not compatible with FF ver 4.0

Did you try starting Firefox in safe mode; this disables the add-ons?

Also I would recommend that you try downloading Firefox 4 again and save it to a new location in case your first download is corrupt.

After downloading, disconnect from internet. Scan the download with your security software and then turn off ALL other programs including security software, choose restart the software after reboot if your security software has such setting. Install the Firefox 4.0 and then reboot your computer. Make sure all security software is up and running, reconnect to internet and test Firefox.

Firefox SafeMode? - A troubleshooting mode.

1.You can open the Firefox 4.0 SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut to open it again.

For more help, see this: http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

Yes, the problem was with the extensions and add-ons. I disabled them all in safe-mode, closed and restarted FF, and ...all is now well. I download extensions one at a time and try them out to avoid the problem.

Thanks.