Some users might want to run the ESR 52.9 release if they still require MPEG-2 playback.
1.) Download Firefox ESR 52.9 disk image: https://ftp.mozilla.org/pub/firefox/releases/52.9.0esr/mac/en-US/Firefox%2052.9.0esr.dmg
2.) Double click the downloaded file to mount the .dmg.
3.) After downloading ESR, drag the Firefox.app bundle to the application folder. (You may need to remove an existing version of Firefox Quantum by dragging the .app bundle from Applications folder to the trash to uninstall it. If you want to run both Firefox Quantum and Firefox ESR 52.9 side-by-side please consult this guide instead: Install Main and Extended Support Release (ESR) branches side-by-side on MacOS
**** BE SURE TO DISABLE AUTOMATIC UPDATES IN FIREFOX 52.9 ESR ***
Launch the app, go to Firefox -> Preferences -> Advanced -> Update and select "Never check for updates (not recommended: security risk)."
It is advised to use the up-to-date Firefox Quantum for normal web browsing and use 52.9 ESR only with our web player and other trusted sites as it stopped receiving backported security updates and fixes as of ~9/5/2018.