I'm not sure why, but even on Windows 11, Firefox says it is running on Windows 10. This is the current "user agent string" for Firefox 136 running on 64-bit Windows 11:

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:136.0) Gecko/20100101 Firefox/136.0

A lot of that is legacy text retained for web compatibility reasons, like Mozilla/5.0 and Gecko/20100101, and the redundancy between Win64 and x64. It's not meant to be too precise.

Two reasons. The main reason is Windows 11 is basically based on NT 10 https://en.wikipedia.org/wiki/Windows_NT#Releases and the second is Mozilla is making the UA's be more generic in fingerprinting.

The Firefox UA has not shown the minor version update since the Firefox 16.0.2 Release https://website-archive.mozilla.org/www.mozilla.org/firefox_releasenotes/en-us/firefox/16.0.2/releasenotes/

User Agent strings for pre-release Firefox versions now show only major version (728831)

The UA also no longer shows if you are using 32-bit Firefox on Windows and Linux, forget which Firefox version that started.

I want to use Firefox and only Firefox on Windows 11. How do I do that?

I want to use Firefox and only Firefox on Windows 11. How do I do that?

Windows will send links from other applications to your default browser, although special steps might be needed for Microsoft Office Outlook if you use that application (see: Setting Firefox as the default browser does not work - What to do). But Windows will use Edge if you run web searches through the system search bar.