Someone reported that Google Hangouts couldn't be full screen while using Windows Firefox, so I took out the EC2 instance I used to try to debug, and found that the full screen function was not called at all...
In the end, I look up their obfuscated JS code for a long time and found that the code directly wrote: If it is Firefox and Windows, it will not call the full screen... Simply...
https://bugzilla.mozilla.org/show_bug.cgi?id=1519051#c4