I have the same problem. If I use Xorg-7.6-3d, then firefox and thunderbird cannot work, the screen goes black, the mouse can't move and I cannot even get to console. If I use Xorg-7.6 instead, everything goes fine. I think it may be the driver problem. I have Lenovo ThinkPad E420 with intel/ATI switchable graphics. However, I can only use the integrated intel graphics so far. I tried the ati-fglrx and the driver given by amd, but both cannot work, just give "failed in waitforX". I don't have xorg.conf in /etc/X11 unless I use the ati driver. After deleting xorg.conf, I can start X, but the driver isn't working. I don't know how to configure. Sorry for my poor English.