You can install multiple versions but there are a few steps to take. When you install the other version(s) of Firefox use the custom installation option and choose a different location to install Firefox. In order to use multiple versions, you need to create a new profile for each version (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 upwards) ----- An alternative approach is to keep Firefox 5 and install the portable version of Firefox 3.6 on your hard drive. The portable version will be totally independent of the main version and can be used when you want to access that site. You can get it from http://portableapps.com/apps/internet/firefox_portable

IMO, your best option is Firefox Portable, available here: <br /> http://portableapps.com/apps/internet/firefox_portable/localization#legacy36