Comment 6 for bug 68291

If you download xserver-xorg-video-tdfx_1.2.1-5_i386.deb from Debian Unstable and replace /usr/lib/xorg/modules/drivers/tdfx_drv.so with the one contained in this package, then tdfx will work correctly except with no OpenGL. I'm not suggesting that this is the fix, but it does show that the problem is likely in Ubuntu Edgy's xserver-xorg-video-tdfx package.