Do you have that problem when running in the Firefox SafeMode?
http://support.mozilla.com/en-US/kb/Safe+Mode
Don't select anything right now, just use "Continue in SafeMode."

If not, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...

I re-downloaded and re-installed Firefox and that seems to have fixed the problem. If you go to this page

http://www.mozilla.com/en-US/firefox/fx/?ref=logo

there is a link on the bottom left corner of the page that says "If you need a fresh new copy of Firefox, you can get it here". Everything has been working better since I did this.