In Firefox 3, images with transparency are not displayed. This appears to be an XAA bug. The problem exists across hardware types and across drivers (vesa, nv, radeon, ...) Adding option "XAANoOffscreenPixmaps" fixes the problem on all systems.
The corresponding Mozilla bug with lots of extra info is:
The Mozilla bug was recently closed as WONTFIX because XAA is supposedly becoming superceded by EXA, but FF3 will soon be running on many systems that have not yet transitioned to EXA, so it still makes sense to fix this in xorg. Thanks!
I think this is a dup of bug 15098.
*** This bug has been marked as a duplicate of bug 15098 ***