OK, I have a weird one. I followed Firefox 4 through the betas, which meant that I had Firefox 3 installed concurrently. Recently, I tried to do a fresh install of Firefox 4 (after removing the betas). First, I tried it as an upgrade. Then I tried uninstalling FF3 and then installing 4. Each time it would successfully install 4, work for a bit (either until I closed it or restarted I am not sure which). Then upon running firefox, it would give me a warning about a restart being needed to completely an upgrade. If I restart the computer, FF4 is no longer installed, having been replaced by the most current version of firefox 3). I have checked the folder and the only things left are the plugins folder and one other folder (don't remember the name). Anyone have any clue on what is causing this?