Firefox 116.0 and later requires macOS 10.15 or later to run. For the eol macOS 10.12, 10.13, 10.14 Mozilla is still supporting the older Firefox 115 ESR which is currently at 115.20.0esr though unless Mozilla decides to extend updates again, the last planned up[date will be 115.21.0esr in March. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support Firefox 115.20.0esr.dmg https://archive.mozilla.org/pub/firefox/releases/115.20.0esr/mac/en-US/ other language folders at https://archive.mozilla.org/pub/firefox/releases/115.20.0esr/mac/ While you may know how to install Firefox on macOS I will still link https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac