hello miklewis, you can try to use portable versions of older versions of the browser & set them to not update (those are not provided by mozilla directly): http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./

You really need a separate Profile for each version of Firefox you have installed, to avoid updating issues and triggering a "scan" of addons every time a Profile is launched with a different version. Also, the preference for turning off updates resides in the Profile, so you can't have it both ways in one Profile. <br />https://support.mozilla.com/en-US/kb/Profiles The easiest way of having multiple versions of Firefox installed is to use multiple older versions Firefox, Portable Edition installed. <br /> http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./ '''P.E.''' packages the "Profile" with the program files installation, each version automatically has its own Profile without the user needing to do anything. P.E. also avoids "competition" for which version of Firefox is the "default browser" in Windows; PE can't be set as the "default" as no registry entries are made. The biggest "downside" is that Plugins need special treatment to work, specific DLL's need to be copied to each PE installation. <br /> http://portableapps.com/forums/support/firefox_portable

Thank you both!