"If true, the resistFingerprinting preference makes the browser report generic spoofed information for data that's commonly used for fingerprinting. Such data includes the number of CPU cores, precision of JavaScript timers, and the local timezone. It will also disable features that are used in fingerprinting, such as GamePad support, and the WebSpeech and Navigator APIs."
And well, apparently it has a pretty bad bug. It spoofs my screen size from previously 1366x768, which 1 in 9.43 browsers have, to 1318x686, which by itself is already unique, because that really is just not a screen size that anyone sells.
So, that's a thing. Maybe it's just isolated to Nightly, but if in doubt, maybe also just test yourself. Now where to report this...
Is it sticky though? I don't know how the spoofing works, but if the next time, the same site gets a different size, wouldn't your profile become less identifiable across visits and other web sites.
No, it changes every time...