I get a similar problem as shown in 
https://bugzilla.redhat.com/show_bug.cgi?id=522763 
the screenshot "Display of firefox when scrolling window with mouse "

In firefox it seems to happen only when smooth scrolling is enabled, and I
am not using compiz. it generally happens when it is partly covered. i also
had this problem in the help viewer.

xorg-x11-server-Xorg-1.7.7-26.el6_0.3(x86_64)
xorg-x11-drv-intel-2.11.0-7.el6(x86_64)
kernel-2.6.32-71.24.1.el6(x86_64)

If i use compiz i do not have this problem

Any suggestions

Thanks

Jon