You can install them both but there are a few steps to take. When you install Firefox 4 use the custom installation option and choose a different location to install Firefox. In order to use both versions, you need to create a new profile for Firefox 4 (Firefox stores user data in the profile). 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)

Another way of installing Firefox 4.0 while keeping the older version of Firefox, is to install Firefox Portable to your hard drive. It will automatically create its' own Profile and won't affect the regular Firefox installation in any manner. http://portableapps.com/apps/internet/firefox_portable