This has been experienced by several users, including me. The problem seems to be that we're using the xorg driver for NVIDIA or ATI graphics. If people switch to the proprietary drivers, the problem goes away. However, the proprietary drivers don't work with some NVIDIA and ATI graphics cards/chips, so if this junk rendering is really bothersome, we'd have to change to the xorg vesa driver.
The problem does not happen with Opera. If your problem is the same as mine, it has nothing to do with being optimized for IE. It's a problem with the xorg driver for NVIDIA and ATI.
SeaMonkey now uses the same base as Firefox, so both of them display the problem.