xorg crashed while using firefox

[Martin Wolf]

while i was closing some tabs in my firefox browser the screen went black and after a few seconds i was presented with a login screen.

i upgraded to 15.10 4 hours ago and i never experienced this behaviour on my 15.04 installation.

i supply an xorg log with the crash in addition

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: xorg 1:7.7+7ubuntu4
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu4
Architecture: amd64
CurrentDesktop: XFCE
Date: Sat Oct 31 03:13:31 2015
InstallationDate: Installed on 2015-08-21 (70 days ago)
InstallationMedia: It
SourcePackage: xorg
UpgradeStatus: Upgraded to wily on 2015-10-31 (0 days ago)