You appear to be using Firefox 115 ESR on Windows 8.1 just fine? For Windows 7, 8, 8.1 the most current version you can run is Firefox 115 ESR currently at Fx 115.20.0esr and the last planned update will be Fx 115.21.0esr in March unless Mozilla decides to extend the updates again before then. https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support Firefox 116.0 and later requires Windows 10, 11 to run. https://archive.mozilla.org/pub/firefox/releases/115.20.0esr/win64/