Hi

For windows 8.1 ?

Mozilla has been still providing security updates for your system through a version called Firefox 115 ESR (Extended Support Release).

Go to Menu > Help > About Firefox

Once that window is open, Firefox will automatically start searching for the latest update available for Windows 8.1.

If it finds one, it will download it immediately.

Update Firefox to the latest release

If you are indeed using Windows 8.1 then the most current version you can use is Firefox 115 ESR currently at 115.34.0esr. 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.