Sometimes, Firefox 4 can’t use hardware acceleration. Usually, this happens because Firefox has identified your graphics card (GPU) or driver (the software that runs the GPU) as being unreliable.
To tell if you’re using hardware acceleration in Firefox, enter the address about:support into your address bar. At the bottom of that page is a section labeled “Graphics.” If you’re using hardware acceleration, this section will contain a message similar to “GPU Accelerated Windows: 1/1 Direct3D 10″.
However, if hardware acceleration is disabled automatically by Firefox, the about:support Graphics section will simply display the message “GPU Accelerated Windows: 0/1.” (Frequently, issues with graphics drivers are resolved by updating those drivers. Contact your computer or graphics card manufacturer for more details.)