https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support Firefox 116.0 and later including the current Firefox 149.0 release and 140.9.0esr requires Windows 10, 11 to run. For the old EOL Windows 7, 8, 8.1 the most current version to (still) support is Firefox 115 ESR currently at 115.34.0esr. It is based on the old Firefox 115.0 Release but with security/stability fixes since and no new features. Go to https://www.firefox.com/download/all/desktop-esr/ > pick 64-bit or 32-bit Windows then you choice of language and then click on [Download ESR 115 ] to download the current Fx 115.34.0esr.