Firefox 116.0 to current requires macOS 10.15 or later to run. Firefox 79.0 to 115 ESR requires macOS 10.12, 0.13, 10.14 to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support For the old eol macOS 10.9, 10.10, 10.11 the most current Firefox you can run is the old Firefox 78.15.0esr which was released back on October 5, 2021 and no updates for it since. https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr If your hardware allows for an update to macOS 10.12 then you would be able to run Firefox 115 ESR which is currently at 115.19.0esr and meets the minimum requirement for this root certificate thing. Information about the root certificate expiration is in https://support.mozilla.org/en-US/kb/root-certificate-expiration Firefox 78.15.0esr will likely still continue to work as it works now. Just that sites that work in more current versions may not work correctly or at all in Firefox 78. Also you likely could not view DRM content anyways.