Using Mozilla Firefox's autoscrolling feature (middle mouse button) with
xcompmgr running results in visual artifacts. This is somewhat similar to the
bug at "http://freedesktop.org/bugzilla/show_bug.cgi?id=1159" (sorry, I don't
know how to make links in Bugzilla). The verdict on that bug was that it was a
fault of the NVidia drivers...however, both my computers have ATI cards and use
DRI drivers.

Computer 1:
     Amd64 (64-bit xorg and firefox)
     xorg 6.7.99.904
     Radeon 8500
     Firefox 9.3
     Gentoo 2004.2

Computer 2:
     Pentium 4
     xorg 6.7.99.903
     IGP 340M graphics card
     Firefox 8
     Gentoo 2004.2

Steps to reproduce:
     Run xcompmgr and Firefox (in any order)
     Ensure autoscrolling is enabled in Firefox's prefs
     Middle-click somewhere on a page, scroll up, down, or sideways to get
artifacts.