Nope, as it turns out this was actually a thing until 2025-01-24, where a commit removed this "pretend to be Windows even on Linux platforms" behavior.
https://github.com/mozilla-firefox/firefox/commit/eb2f90f870...
This OS spoofing behavior was added in 2019-01-09 with this commit:
https://github.com/mozilla-firefox/firefox/commit/264fe08c09...
So Firefox has spoofed the User-Agent as a Windows machine on Linux for around 6 years, and only stopped doing it early this year. Would love to see whatever gaslit you into forgetting this easy to test and validate behavior.
https://support.mozilla.org/en-US/kb/resist-fingerprinting
This was part of the resist fingerprinting feature. Which is an advanced user configuration. I can alter the user agent directly myself too.
Sigh
I regret getting tricked into arguing over such a pedantic specific, So I'd like to redirect the actual point, which is that it's not meaningful if a Firefox browser pretends to be a slightly different Firefox browser, but instead the problem is when something that's not a browser, claims to be and behave like one.
Still, +1 for finding the commit, I'd forgotten about this feature. I thought only the tor browser was this foolish.