Some users might want to run the latest release of Firefox alongside the ESR 52.9 release if they still require MPEG-2 playback. Note: since the two apps share a name, you can only have one of them open at a time.
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 out of the read-only .dmg and onto your desktop.
4.) Drop focus onto the .app bundle's icon to select it and press enter once. This will trigger edit mode, so you can rename the .app bundle. Rename it to "Firefox ESR" and drag this renamed bundle onto the .dmg's shortcut to Applications.
5.) That's it - you now have the two .app bundles side-by-side in Applications.
**** 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.