Originally Posted by danix
Firefox sometimes allocates large pixmaps which exceed the hardware limitations of the GPU. These pixmaps have to be handled by a slow software fallback.
I don't pretend to be an X expert, but to my little programming mind it would make sense to break up a 'too large' piece into pieces the GPU can handle, either in GPU memory or in system memory, and allow the GPU to process it. Most likely how the open source driver gets away with it.
Of course that would allow older cards to continue to work without requiring someone to spend money on a new card.