Earlier quoted context omitted.
That just means your Firefox isn't benefiting from GPU acceleration. It could either be because your OS or hardware doesn't support it or because your graphics drivers are out of date.
The reason why is unimportant. The end result is the same: terrible performance on Firefox, good performance on Chrome. It's not up to the end user to know or care why.
Terrible on YOUR machine. On something like 45% (and increasing) of vista or win7 machines the story is the opposite.