''<p>llkabong [[#question-1362408|said]]</p>'' <blockquote>I am using El Capitan because I am using an old Mac. Operating system 10.11.6. I've tried downloading FESR but always get the same error (pictured). What is the problem? </blockquote> The problem is you are using a too old Mac OS version which is 10.11.6 (ten.eleven.six). Firefox since Fx 79.0 and later including the current Fx 95 and 91 ESR requires Mac OS X 10.'''12''' (ten.twelve) at minimum. Besides updating your OS to Mac OS X 10.'''12''' or later, the most current Firefox version you can install on mac OS 10.11 is Fx 78.9.0 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/95.0/system-requirements/ 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/