By default, Firefox 4 beta will use the same profile folder (location where bookmarks, passwords etc are stored) as Firefox 3.6 If you will only be using Firefox 4 beta this is not a problem, but if you plan to switch between the 2 versions you need to create a new profile for Firefox 4. Using the same profile with different versions can cause problems. Once you create a new profile you can copy certain data such as bookmarks and passwords between them. You can also use Firefox Sync to keep the profile synchronized. Finally, you will need to set up shortcuts to launch the correct profile for each version. The following links show how to do this: * http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows * http://kb.mozillazine.org/Shortcut_to_a_specific_profile * http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox * https://addons.mozilla.org/firefox/addon/firefox-sync (only needed for Firefox 3.6, it is built into Firefox 4)