Download Firefox to an old Mac
Firefox says my Mac needs os 10.12 or later. My machine is only 10.11 something. What can I do? (My machine can't take newer versions of Safari)
Wšě wotmołwy (1)
The latest version of Firefox that still supported the old EOL Mac OS X 10.9, 10.10, 10.11 was 78.15.0esr as 79.0 and later has required 10.12 at minimum to run.
https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr
You can download the Firefox 78.15.0esr.dmg from 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/