This Mozilla developer is currently investigating the graphics driver black-list for Firefox and deciding which drivers should be allowed for future Firefox releases. With that said, he's also looking at possible enabling LLVMpipe as a means of Gallium3D software rendering within Firefox.
LLVMpipe is the the Gallium3D Mesa driver that takes advantage of LLVM for providing CPU-based optimizations as a much better means of software rendering on the CPU than the Mesa classic software rasterizer or Gallium3D's "Softpipe" alternative. LLVMpipe is able to take advantage of multiple CPU cores, the latest CPU instruction sets, and other processor features.
This open-source graphics driver is still far from ideal for running games, but it can work for some with the latest and greatest Intel/AMD processors. LLVMpipe though is sufficient for running GNOME Shell. For basic WebGL within Firefox, it will likely work out quite okay too.
However, due to the recent EGL upbringing and other work, this enabling of LLVMpipe inside Firefox would likely be limited to use on Mesa 8.0 and later.
Part of the discussion can be found from this mailing list post.