You can update from a Firefox 3 version to Firefox 4.0.1.<br /> There is no need to uninstall Firefox 3, but you can do that to prevent leftover entries in the registry that aren't removed with updating and files in the Firefox program folder that are no longer needed and used. Do not remove personal data if you uninstall the current version. Your bookmarks and other profile data are stored elsewhere in the [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder] and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.