Link to LP bug: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1530704 firefox/ chromium and all other programs freeze.
https://launchpadlibrarian.net/232773187/XorgLog.txt shows the radeon driver is in use and many mieq overflows.
Would be interesting to see a proper backtrace of where Xorg is hanging. See http://wiki.x.org/wiki/Development/Documentation/ServerDebugging/ for some background info.
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