Firefox 116.0 and later including the current versions requires macOS 10.15 (ten.fifteen) at minimum to run. Since you are running the old eol macOS 10.11.5 the most current Firefox you can run is Firefox 78.15.0esr as Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr If you can update your macOS to at least 10.12 then you can run the still currently supported Firefox 115 ESR. It is currently at 115 19.0esr and will be updated til 115.21.0esr unless Mozilla decides soon to continue updates again. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support