UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:49.0) Gecko/20100101 Firefox/49.0

Steps to reproduce the problem:
1. open webRTC data channel
2. add window.beforeunload() a function with debug statement in first line. 
3. see data channel and peerConnection are already closed when debug statement is reached. 

What is the expected behavior?
Should close after beforeunload and thus be open in beforeunload. Clearly is not beforeunload but afterunload regarding peer Connections; contradicting documentation.  

What went wrong?
peer connection and data channel closed before beforeunload when should close after beforeunload as means user cannot chose to stay on page if, for example, it is an important page. 

Did this work before? N/A 

Chrome version: <Copy from: 'about:version'>  Channel: n/a
OS Version: 
Flash Version: Shockwave Flash 11.2 r202