Firefox 78.15.0 is the last version for your Mac OS X platform (update channel: mozilla-mac-eol-esr1) and you will no longer receive updates, so you are out of luck if you can't update your OS. Current releases require Mac OS X 10.12 or newer. *https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr Firefox 78.15.0 on the Mozilla CDN server: *https://download-installer.cdn.mozilla.net/pub/firefox/releases/78.15.0esr/

https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales are at https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/