Reproduction steps:
1. Build the attached project to WebGL
2. Open the built index.html file in Firefox
3. Open the developer console to see the output (F12)
4. Click within the WebGL canvas while the Unity loading progress bar is shown.

Result: notice how there is no OnApplicationFocus callback. If you click the mouse button again then the callback will occur, but it doesn't occur until you click the mouse.

This is incorrect because the WebGL canvas actually does have focus and can receive mouse movements before you click a second time.

Reproducible on: 5.5.1p2, 5.6.0b7