https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr For macOS 10.9, 10.10, 10.11 users the most current Firefox with security updates that you can run is Firefox 78.15.0esr, which you may have. Firefox 79.0 to 115 ESR requires macOS 10.12, 10.13, 10.14 at minimum to run while Firefox 116.0 or later requires macOS 10.15 or later to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support If your hardware does support updating to at least macOS 10.12 then you would be able to run Firefox 115 ESR which is currently at Fx 115.18.0esr and will be updated to 115.21.0esr in March 2025. Mozilla will decide in early 2025 on whether to continue with further updates as 115 ESR was originally going to end at Fx 115.15.0esr.