Have a look. https://support.mozilla.org/en-US/questions/1202522

Firefox 45.9.0esr is the most current you can run on the ancient macOS 10.6, 10.7, 10.8 https://support.mozilla.org/en-US/kb/firefox-osx Firefox 78.15.0esr is the most current you can run on macOS 10.9, 10.10, 10.11 https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr Firefox 116.0 and later requires macOS 10.15 (ten.fifteen) at minimum to run so the macOS 10.12, 10.13, 10.14 users are being served the Fx 115.0 ESR (currently at 115.7.0esr) until 115.15.0esr in Sept 2024 in order to have security and allowed stability updates. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support