You can't get around giving nearly the entirey of the User Agent string simply thorugh feature detection (well, except for the phone model when on mobile). Canvas and WebGL can be fixed by limiting their functionality, or at least that's what Firefox does in the aforementioned mode. Canvas could conceivably be fixed to just behave the same everywhere, but for WebGL you have to limit it to some common denominator to hide the GPU model, so privacy nessesitates less functionality. The list of plugins can be spoofed to be empty. Screen resolution (or more importantly, window size) has to be somewhat accurate because lots of websites use it for responsive design. Firefox can round the window size to the nearest multiple of 200x100 which doesn't break too many things, but that's still far from giving the same value for everyone.

https://wiki.mozilla.org/Security/Fingerprinting