No guarantees this will fix your problem, but I too just experienced the same thing. Try this: while running firefox4, open your "tools" menu and go to "add-ons" and open it up. Check the version of Java you're using. If you are not using v6.0.24 upgrade to that version. After you've upgraded, open your "add-ons" menu again and delete any of the previous versions listed. One other thing, my problem cropped up after installing the Yahoo tool bar... Don't know if this was part of the problem or not, but I uninstalled it, and did the Java upgrade, then deleted the previous two versions in my add-ons and the problem went away. I hope this helps, and good luck!