You appear to be using the old EOL macOS 10.11 so Firefox 78.15.0esr is that most current Firefox version you can run on it. Firefox 78.15.0esr is the latest to still support the old EOL macOS 10.9, 10.10, 10.11 https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 at minimum to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support Firefox 116.0 and later requires macOS 10.15 or later to run. If you can update to at least macOS 10.12 (ten.twelve) then you can use the still currently supported Fx 115 ESR (115.8.0esr) as the last update 115..15.0esr will be in Sept 2024. https://www.mozilla.org/firefox/all/#product-desktop-esr (<-- requires macOS 10.12 or later as it will not run on your 10.11) If you got a email about updating Firefox, the email was to say Sync was not going to be supported anymore in the old Firefox 78.0 and earlier versions as you need 79.0 or later to use Sync. https://support.mozilla.org/en-US/kb/sync Firefox 78.15.0esr can be used just the same after March as you can now minus the ability to use the Sync feature. It is also the most current web browser you can use on your old EOL macOS 10.11 so your options is limited if you cannot update your macOS to at least 10.12.