Firefox 116.0 and later including the current Firefox Releases and 128 ESR requires macOS 10.15 (ten.fifteen) or later to run. If your hardware allows you to update your macOS to 10.15 or later then you will be able to use current versions of Firefox. At the moment the last planned update for the old Firefox 115 ESR will be 115.21.0esr unless Mozilla make a decision before then to extend updates again as Firefox 115 ESR was originally going to end at Fx 115.15.0esr. If they do decide to extend updates again it may only be for another some six months like the current updates extension. If Firefox 115.21.0esr is indeed the last update then it will still continue to work. Just will not get anymore security and or stability updates. The Chrome/Chromium based browsers have already dropped support of the eol macOS 10.12, 10.13, 10.14 a while ago.