Description of problem:
When switching windows while having only firefox open, the system crashes, and automatically logs out. 

However, when having multiple applications running, or an application other than firefox, the system doesn't crash when switching windows. 

Version-Release number of selected component (if applicable):
50.1.0

How reproducible:
Very easy

Steps to Reproduce:
1. Start firefox.
2. Open multiple windows in firefox.
3. Make sure firefox is the only application running.
4. Switch windows (for example by Alt + `, or some custom shortcut).

Actual results:
System crashes, and logs out.

Expected results:
Switch windows in firefox without crashing.

Additional info: