Firefox hangs after answering yes to Firefox 4 upgrade

Question

Today when My wife started Firefox from her account she was presented with a window asking if she wanted to update to Firefox 4. She answered yes. At that point Firefox (v3.6.16) became unresponsive with no indication that anything was happening. After some time she tried to shut down Firefox by clicking the X in the upper right corner of the window. She then got the Windows End Program box stating that the program was not responding and allowing her to shut the program down.
She then restarted Firefox and after what seemed to be a normal start up with the home page being displayed, Firefox would again hang after selecting a bookmarked site.
When my wife called my attention to this problem, I decided to try it from my Windows XP account to see if the problem only happened in her account. When I first opened Firefox, I briefly saw a progress bar indicating Firefox was being updated. Firefax then opened and loaded two tabs, my home page which is set to the google.com/firefox start page and the same Firefox 4 anouncement page that my wife got when she first started Firefox. When I then selected most bookmarked web sites or even the Firefox Support link on the home page, Firefox would hang and require ending the task to exit. Terminating Firefox in this manner, and restarting Firefox produced the same result. Rebooting the PC after terminating Firefox also produced the same result. Using IE8 to visit the same web sites was successful and resulted in no hangups. In fact, I am currently using IE8 to post this question.
After restarting and trying many web sites, I did find some sites that opened normally. One was the About Mozilla link on the home page and another was a local weather site: wunderground.com.
I also ran Firefox in safe mode from the command line and got the same hanging which I understand indicates that the problem is not being caused by any plugins or extensions.
Any suggestions as to how I can fix this problem would be greatly appreciated.

Answer 1

Thanks for the suggestions. As it turned out Firefox seemed to fix itself!!! First I must add that I don't believe that my wife clicking on the Firefox 4 update button had anything to do with the hangups. There was no indication that anything had been downloaded. Firefox would start normally but hang when trying to load most web sites. Usually a progress bar would be displayed at the bottom of the page that would never completely load. Clicking on the "Stop loading this page" button would only result in the Firefox symbol at the top left corner turning to a white square and Firefox would become non-responsive requiring an end task to shut down. It acted like an Internet problem but as I reported before, when I tried the same sites in IE they loaded properly. Also rebooting had no immediate effect.
As I kept trying in my account, more and more web sites started to load properly. After a couple of days, there were no more hang ups occurring at all in my account. The only changes I had made was to disable a couple of extensions: DownloadHelper and Freecorder Community Toolbar in my account. These were never installed in my wife's account and had no immediate effect after disabling them. Unfortunately, Firefox would still hang in my wife's account. She had changed the default browser to IE and when I opened Firefox in her account I got the message box asking if I wanted to change the default to Firefox. At that point clicking on anything resulted in a hang as before. I even tried setting the default browser back to Firefox in Windows "Set Program Access and Defaults" and rebooting but got the same result when I tried running Firefox again.
Now, after doing nothing to Firefox for a couple of days, Firefox is working correctly in both accounts??? I did notice a lot of hard drive activity during the time I was working this problem. Hmmm.
I use Norton 360 v4 as my security protection on this machine. They have now released v5 which I am about to upgrade to. I wonder if their LiveUpdate was doing something that affected Firefox?
I think I will wait awhile before trying to update to Firefox 4.