https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release https://www.mozilla.org/en-US/firefox/organizations/

https://www.mozilla.org/en-US/firefox/organizations/notes/

https://www.mozilla.org/en-US/firefox/organizations/system-requirements/

Thank you FredMcD! This is the error I'm getting now:

Your security preferences allow installation of only apps from the App Store and identified developers.

See: https://support.accessdata.com/hc/en-us/articles/204329335--Your-security-preferences-allow-installation-of-only-apps-from-the-Mac-App-Store-and-identified-developers-

http://www.cultofmac.com/224876/safely-install-non-mac-app-store-apps-on-your-mac-os-x-tips/

Note that you need the Firefox 52.1.0 ESR version.

• https://www.mozilla.org/en-US/firefox/organizations/all/

Installing multiple versions on Mac OS X is basically dragging the Firefox application out of the DMG file to the desktop and rename the folder. You can drag the renamed Firefox application folder to the Applications folder like you would normally install Firefox.

You need to use the terminal to create a new profile for that version. Make sure that no profile is selected as the default to make the Profile Manager show when you start Firefox. You can check that in the profiles.ini file.

• http://kb.mozillazine.org/profiles.ini_file
• http://kb.mozillazine.org/Profile_Manager

You should be able to use the Extended Support Release (ESR) of Firefox 52 -- it will still run Java and the other old plugins that ran in Firefox 51. ESR was first developed to meet the needs of large organizations that wanted Firefox to change much more slowly, but anyone can use it. It gets security updates but no feature changes for about a year at a time.

More information on this change and ESR: Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?