-e: with the introduction of webglContextAttributes option for overriding said attributes the "don't clear" flag on the camera can be made working as expected. but it fails to do so on Firefox on Windows.

-repro:
--Download the project and open "test" scene.
--Make sure main camera's Clear Flags option is set to "Don't Clear" or "Depth only".
--Play scene in editor, move cube with left/right arrow keys.
--Observe the the previous draws of the cube sticking around looking like a white line
--Build for WebGL
--under <build>\Build edit the json as follows:
---set "preserveDrawingBuffer" from false to true
--run build in FF on Windows

Expected Result: Current frame should be rendered on top of previous one. (As seen in editor.)
Observed Result: the cube doesn't move at all (see console that the left/right commands are actually triggered)