Same problem here. I installed version 4.0, then removed it and tried to install 3.6. Now no version Firefox will not install. I keep getting a message that I have to restart my computer in order for updates to a previous version can be installed. Every time I restart the computer and try to install firefox the same message keeps appearing. I attempted to uninstall the earlier version of firefox using "programs and features" in windows but the same message came up and thus I was unable to uninstall using that method.

I have managed to cure the problem in the following manner;

1. Click "Start" 2. Click "Computer" 3. Click on "C" drive ( the one on which your operating system is installed). 4. Click on "Program files (x386)" 5. Scroll down to the "Mozilla Firefox" folder. 6. Right click on the folder and select "Delete" and delete the folder. 7. Re-install Firefox.

I had expected to have lost all my Bookmarks but was pleasantly suprised when they all re-appeared after the re-install.