We currently don't have a way of exposing (mostly-)raw mouse movement/position data. This is very useful for improved smoothness when using mouse coords/movement. This is particularly important for games, as better-than-pixel-perfect mouse positioning is necessary to get a smooth feel, and, particularly for FPSs allow for very fine aiming. Without better precision for mouse coords/movement, some users will have deadzones where they cannot click/move/select/aim/etc. For a 3d view, sometimes these deadzones are as bad as multiple degrees, which causes stuttery rotation and frustration with a 'jumpy' camera. This seems like it would be most easiest to integrate with pointerlock, but is also possibly analogous to what was needed for high-precision timestamps.

Note that if integrated with pointerlock it should be additional to movementX/Y (like rawX/Y) because the mouse is also subject to WM accelleration etc. which is equally undesired for the raw mouse input as pixel clamping.

Jeff (or Florian?) - would either of you be willing to mentor a community member through this?

I do not have any expertise regarding how to do this in Gecko or WebAPIs, so I don't think that I can 'mentor' here.