Reverting back to Firefox 2 is not simple, Firefox 3.6.13 stores its data such as bookmarks and passwords in files that Firefox 2 can not use. If you just need to use Firefox 2 for that one site, an alternative option is to install Firefox 2 portable version on your hard drive and use that for the one site. The portable version will be totally separate from Firefox 3.6.13 which you can use for your normal browsing. You can get Firefox 2 portable from http://portableapps.com/apps/internet/firefox_portable - the download link for Firefox 2 is near the bottom of that page in the "Legacy Versions" section. When you install the portable version, it will ask where you want to install it, choose a location on your hard drive. When you run Firefox 2 portable it will create its own profile folder (location where Firefox stores user data), and your profile folder for Firefox 3.6.13 will remain untouched.