My only complaint with Mull is the 60hz lock. I know it's inevitable if you want 'resist fingerprinting' enabled, but it is really noticeable after using 120hz for a while.
I can't help but think it should be possible as it's a client-sided issue, and there is no reason the browser can't simply advertise itself as 60hz but actually allow 120hz scrolling, but this is something for Mozilla to fix.