Firefox since Fx '''79.0 and later''' including the current Fx 99 and Fx 91 ESR requires macOS '''10.12''' (ten.twelve) at minimum. Besides updating your macOS to macOS '''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/98.0/system-requirements/ '''In case you need the Firefox 78.15.0esr file''' see below links. 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/