52.7.3 ESR. It will continue to get security updates until May 2018, and you can download and install it from this page:

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

Note : Legacy Extensions will be deleted or removed in any version update after June 2018. Where it will become Quantum based.

You should make a backup of your Profile before going back and just because:

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Please let us know if this solved your issue or if need further assistance.

GratefulTone said

I have an older Mac system. OSX 10.7.5 I can no longer use Youtube and Firefox. I can not upgrade FireFox. Do you have any solutions.

https://support.mozilla.org/en-US/kb/firefox-osx - Firefox support has ended for Mac OS X 10.6, 10.7 and 10.8

Firefox 45.9.0esr will be the most up to date version with security fixes that will run on OS X 10.6, 10.7 and 10.8 as Firefox 49.0 and later requires OSX 10.9+

http://archive.mozilla.org/pub/firefox/releases/45.9.0esr/mac/ http://archive.mozilla.org/pub/firefox/releases/45.9.0esr/mac/en-US/

The Firefox 48.0.2 was released August 24, 2016 and is missing several security updates that the Firefox 45.9.0esr released on April 19, 2017 has.

Perhaps using a Linux distro with a lighter desktop manager like Xfce vs a heavy one like KDE or Gnome3 may be of interest.

https://www.macworld.co.uk/how-to/mac/how-install-linux-on-mac-3637265/