I tested it on a fresh profile on Firefox Nightly 59.0a1 (2017-11-21), using https://panopticlick.eff.org/.

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...