The latest release of the old eol High Sierra is macOS 10.13.6 Firefox 116.0 or later requires macOS 10.15 (ten.fifteen) or later to run. For macOS 10.12, 10.13, 10.14 the most current you can run is Firefox 115 ESR which is currently at 115.14.0esr. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support You can get the Firefox 115.14.0esr.dmg at https://www.mozilla.org/firefox/all/#product-desktop-esr as make sure it is Firefox 115,.14.0esr and not 128.1.0esr and pick your language if the selected option is not your choice. You can also download Firefox 115.14.0esr.dmg from below. https://archive.mozilla.org/pub/firefox/releases/115.14.0esr/mac/en-US/ https://archive.mozilla.org/pub/firefox/releases/115.14.0esr/mac/ However unless Mozilla decides to extend support for the older Fx 115 ESR channel, the last update will be 115.15.0esr in early September with no more security and stability updates after. Unless you can update your macOS to 10.15 or later that will be it. Also I moved this thread from Firefox for iOS (for iPhone/iPad) to Firefox section as you have '''macOS'''.

Oh and since you may be new to using macOS, there is the article on how to install Firefox on a Mac. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac