Created attachment 91307 [details]
Xorg log

This crash happen after switch from fullscreen to windowed mode of YouTube
HTML5 video player in Firefox with enabled hardware acceleration (about:config
option layers.acceleration.force-enabled is set true; on Graphics section of
about:support there is 1/1 OpenGL). Like in previous case when that happen
image on screen just freeze, but audio playback is continue.

Radeon HD 6620G
Linux 3.13-rc5
libdrm 2.4.50
Mesa 10.0.1
xserver-xorg-video-radeon 7.2.0
Xserver 1.14.5-1ubuntu2
Kubuntu 14.04 x86_64 Alpha 1 updated today.