High Sierra is macOS 10.13.6. Firefox 79.0 to 115 ESR requires macOS 10.12 (ten.twelve), 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 (ten.fifteen) or later to run. Since you have macOS 10.13 you can use the still supported Fx 115 ESR (currently at 115.10.0esr) as the last update 115..15.0esr will be in Sept 2024. https://www.mozilla.org/firefox/all/#product-desktop-esr I moved this thread from Firefox for iOS (for iPhone/iPad) to desktop Firefox section.