I'm using two monitors with different resolution, the default driver can handle that with no problem, but some applications have painting problems (e.g. Thunderbird cannot paint correctly and I have to hover over the windows a lot to get it to repaint, so I can read the mail subjects again.)
So I installed fglrx
$ fglrxinfo display: :0 screen: 0 OpenGL vendor string: Advanced Micro Devices, Inc. OpenGL renderer string: ATI Radeon HD 5450 OpenGL version string: 4.2.12217 Compatibility Profile Context 12.104
Now Thunderbird works normal again, but all transparency is lost from Unity.
It appears as if I'm running Unity2D now. Why is that and how do I switch to OpenGL version?