If anyone else encounters this, I've found a solution to my problems: http://wiki.archlinux.org/index.php/ATI_Catalyst#Catalyst_10.6.2F10.7_:_black.2Fgrey.2Fwhite_boxes.2Fartifacts_in_firefox.2Fthunderbird
http://phoronix.com/forums/archive/index.php/t-24323.html Apparently it's the new 2D acceleration scheme that's buggy... Best regards Peter K
