There is a version of the Yahoo toolbar for Firefox 4 - https://addons.mozilla.org/firefox/addon/yahoo-toolbar You should check with Zynga to see if they have updates for Firefox 4. To use Firefox 3.6.15 you can uninstall Firefox 4, but when uninstalling do not select the option to "Remove my Firefox personal data" as that option will delete all of your Firefox bookmarks, passwords and other Firefox user data. You can then download and install Firefox 3.6.15 from http://www.mozilla.com