I just did the upgrade from version 12.04 to 14.04 and immediately had some problems.
Whenever I try to open certain webpages using Firefox or Chrome, the screen just freezes. Mouse and keyboard are not responding and shortly after that system automatically logs out.
It's especially problematic with Firefox. Most of the time it freezes right after the launch.
Couple of other programs I have used so far seem to work fine.
I examined the error messages and it seems that there is something going on with Xorg. But I have no idea how to fix it.
So far I've noticed that only the Firefox and Chrome are causing the crash (haven't tested the Opera yet). All the other applications (termial, gedit, Virtualbox, qFTP etc...) seem to run smoothly.
I tried to remove the Firefox and install it again, but it didn't solve the problem.
On one occasion, when the crash happened, similar message was shown on the screen:
INFO: task Xorg:2746 blocked for more than 120 seconds.
So I'm not sure whether or not the Xorg is responsible.