Hey per.billing, There are a lot of benefits to running the latest version of Firefox. You might want to consider running the older version on [http://www.freesmug.org/portableapps on a USB Stick with PortableApps]. That way you'll still be able to run the latest version for your day to day activities. If you still want to roll back to a previous version of Firefox, just take a look at the Knowledge Base article [[Installing a previous version of Firefox]]. Hopefully this helps!

See this article about how to install multiple versions on Mac OS X: * http://www.businessinsider.com/how-to-run-firefox-4-and-3-simultaneously-in-mac-os-x-with-multiple-firefox-profiles-2011-3