Firefox 4: hardware acceleration

Content acceleration speeds up rendering the actual page content, such as the text, images, CSS borders, etc. Content acceleration is also used to speed up the 2D HTML CANVAS.

Compositing acceleration speeds up putting together chunks of already-rendered content (layers) together for final display in the window, potentially applying effects such as transforms along the way. For example, if you had a HTML5 video that was playing, and it had CSS effects applied to it that was making it rotate and fade in and out, that would be using compositing acceleration to make that go fast. (This feature is not activated by default yet.)