As I exit out of full-screen from youtube, the picture on the computer is frozen. I can still hear the audio and the cursor can move but I cannot interact with the OS and the screen seems to be frozen. This has happened several times, it does not happen every time but enough to frustrate me.
I think both Chrome and Firefox use PPAPI, the pepper flash plugin so could be a compatibility issue with that. I would try looking in ~/.xsession-errors or ~/.xsession-errors.old if you have had to reboot.
Also if you have an Intel display adapter then try enabling "sna" as the Acceleration Method, it fixes a lot of video issues with some Intel chipsets in Ubuntu based distro's. Here's a link on how to do it.