In your More System Details on right "User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X '''10.11'''; rv:78.0) Gecko/20100101 Firefox/78.0" Firefox 79.0 and later has required MacOS 10.'''12''' at minimum to run so you will need to upgrade to macOS 10.12 or higher. 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/94.0.2/system-requirements/