MBP keeps crashing when changing users while having firefox open on user B.

Let's say there are two users and one is user A and one is user B.

When user B has Firefox one and switches to user account A, the whole computer crashes and we have to restart.

However it never happens the other way around. User A can have firefox open and switch to user B but computer won't crash.

Also it never crashes as long as Firefox is not open in user B.

 

I noticed that it happens the other way around too.
If I have FF open on one user and switch to another, the laptop crashes and I have to press a key to restart.