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.