If you uninstall Firefox 4 beta, do not select the option to "Remove my Firefox personal data" as that will remove all Firefox user data such as bookmarks and passwords used by all version of Firefox. For details on uninstalling see click here
You may still already have Firefox 3.6 on your computer, as by default the Firefox beta installs in a different location. If required you can re-install Firefox 3.6, download the latest installer from click here and run it to install Firefox. When you re-install Firefox it will use the user data that is already present on your computer.