Your user agent probably shows as Windows 8.

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

It's possible that Firefox is set to run in backwards compatibility mode for Windows 8 or 8.1. In that mode, Windows emulates the old OS and Firefox doesn't know your true OS.

To check on that setting, right-click your Firefox desktop shortcut, click Properties, then click the Compatibility tab. For best results, un-check everything on the tab so Firefox run with default Windows 10 settings.

But... your Firefox reports that it is on the Extended Support Release track. I'm not sure how to easily switch back to the standard release track (running the regular installer may create a separate, parallel installation).

This is your useragent used to post this thread. Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:109.0) Gecko/20100101 Firefox/115.0

The NT 6.2 is Windows 8 as NT 6.3 is 8.1

Windows 7, 8, 8.1 users are being served the Fx 115 ESR in order to still get security and stability fixes till Sept 2024. When you downloaded Firefox from a mozilla.org page, it thought you were on Windows 8 due to the useragent.

I agree with above that you may likely be running Firefox in Windows 8 compatibility mode for some reason vs having a modified ueragent.