Reproduction steps:
1. Open the attached project
2. Attach "TestMouse" script to the Camera gameobject
3. Build the game on "Web Player" Platform and run it on Firefox
4. Press middle mouse button and release it outside of the player
5. Notice that OnGUI still shows MMB is pressed

- Note, could not reproduce this issue on Internet Explorer
- Note, same problem is present with right mouse button, but LMB works fine

Workaround: press MMB/RMB again inside the player

reproduced on: 5.3.4p1, 5.2.0f3, 5.1.4f1