FF + Unity plugin crash when navigating back to game.

Repro Steps:

1. Install 4.0.0 b10 and set channel to beta
2. Launch FF with http://paralympics.channel4.com/boccia/
3. During progress bar -> right click -> Click Setup in context menu
4. Once you see Setup page click the back button in the browser
5. Observe plugin crash during progress bar

Desired outcome: No plugin crash and content should load.

Note: No crash folder generated & nothing helpful in the log file, but FF plugin container crash.dmp is attached along with video and screenshot.