Using Mozilla on Mac El Capitan Operating System

It would be 78.15 ESR. See [https://www.mozilla.org/en-US/firefox/78.15.0/system-requirements/ Firefox System Requirements].

Can you post a download link to that version, please?

https://download-installer.cdn.mozilla.net/pub/firefox/releases/78.15.0esr/mac/en-US/Firefox%2078.15.0esr.dmg

Firefox 78.15.0 ESR is the last version for Mac OS X 10.9/10.10/10.11 (mozilla-mac-eol-esr1) and you will no longer receive updates. *https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr *https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support See also: *https://support.mozilla.org/en-US/kb/install-older-version-of-firefox