I sent an exported HTML game to a friend, who said that 3D mouselook doesn't work on Firefox.
I tried it myself, and it seems to be correct. When using Firefox, X-rotating the camera on mouse movement doesn't work.
What's even stranger is that it works on Firefox when using debug mode in the Construct editor, but not when using preview mode on Firefox without debug.
When in the Construct editor's debug mode on Firefox, capturing the mouse movements seem to work fine as I can easily output the movement values to text, but for some reason it doesn't translate into camera rotation.
I've attached an example which works fine in all browsers except for Firefox. However, it still works in Firefox preview with debug mode as previously mentioned.
Attached example: dropbox.com/scl/fi/ustmlnw42fnwlgrng4wbi/firefoxcamera.c3p