In some cases, your PC vendor will insist on releasing their own modified driver updates and there’s no guarantee they’ll do that. Your graphics card may be WebGL-compatible, but it’ll be blocked in Firefox if the driver version is 0.0.0.1 behind the approved list. Type “about:support” in the address bar and scroll down to the “Graphics” section at the bottom to discover whether your card is suitable.

Type about:config in Firefox’s address bar and make the following changes:

   * To enable WebGL, set webgl.force-enabled to true.
   * To enable Layers Acceleration, set layers.acceleration.force-enabled to true
   * To enable Direct2D in Windows, set gfx.direct2d.force-enabled to true

Did you check the graphics section on the "Help -> Troubleshooting Information" (about:support) page ?