Firefox since Fx 79.0 and later including the current Fx 96 and 91 ESR requires macOS 10.12 (ten.twelve) at minimum. Besides updating your OS to macOS 10.12 or later, the most current Firefox version you can install on macOS 10.9, 10.10 and 10.11 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/96.0/system-requirements/ Firefox 78.15.0esr can be downloaded from 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/