I thought we pinned that down to using an xorg NVidia or ATI driver. The solution was to use either VESA or the proprietary ATI or NVidia driver.
I've had that problem with Firefox and SeaMonkey for a few versions. Both the computers with the problem have an ATI graphics card or onboard chip. For one computer, I can't build a proprietary driver because the card is too old. The other *should* work with the proprietary driver but doesn't and I'd have to build it from source. I really don't need what the proprietary driver would give me, so I just live with the Firefox/SeaMonkey rendering problems. Mostly I use Opera anyway.