I guess you figured out Firefox 78.15.0 ESR was the last version for macOS 10.11 Below is what I was going to post before you edited. Firefox since Fx '''79.0 and later''' including the current Fx 100 and Fx 91 ESR requires Mac OS X '''10.12''' (ten.twelve) at minimum. Besides updating your OS to Mac OS X 10.12 (ten.twelve) or later, the most current Firefox version you can install on mac OS 10.'''11.6''' is Fx '''78.15.0''' ESR though it is EOL with no more updates. https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr https://www.mozilla.org/firefox/78.0/system-requirements/ https://www.mozilla.org/firefox/79.0/system-requirements/ https://www.mozilla.org/firefox/100.0/system-requirements/ '''In case you ever need the Firefox 78.15.0esr file. ''' https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales at https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/