Firefox 4 final will be released with hardware acceleration enabled by default, for Firefox 4 to use hardware acceleration you should be using Direct X 10 supported graphics card. All graphics cards or video drivers doesn’t support hardware acceleration, Firefox 4 has a “black list” of graphic cards or drivers which makes sure Firefox doesn’t use graphics drivers or chips that tend to cause problems.

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Checking  Firefox 4 is using hardware acceleration or not

1.Type about: support in the address bar or from Help menu, click Troubleshooting information.

2. Now at the bottom of the page you can observe Graphics section, if you’re using hardware acceleration, the  Graphics section will display a message as “ GPU accelerated Windows: 1/1  Direct3D 10”. That means your graphic card supports hardware acceleration and Firefox 4 enabled it and automatically using it.

Firefox 4 using hardware acceleration, hardware acceleration is enabled

3.If Firefox automatically disabled hardware acceleration as it detected incompatible graphics card as we said Firefox 4 maintains blacklist of unsupported graphics cards or drivers, then Graphics section in about: support page shows “0/1” for “GPU accelerated windows “ like shown in below picture.

hardware acceleration disabed in Firefox 4

Via [Mozilla blog]