What problem are you having with 4.0.1? It is a security update and it is not recommended to go back to 4.0 If you want to go back to 4.0, first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data. You can then install Firefox 4.0 available from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0/win32/en-US/ (English US version)