This is a rather ambiguous and incorrect statement. Hardware acceleration of some things is disabled by default because of a security issue related to the way X11 works (it's complicated but in short, one window can "see through" to another window--even if it's an entirely different app). You can re-enable it via the prefs (forget the name but it's under "Performance") or just toggle `layers.acceleration.force-enabled` in `about:config`.

Furthermore, acceleration is enabled by default for loads of things. It's just some certain WebGL features that aren't hardware accelerated. For example, when you play back a video it's going to use hardware acceleration by default (if the underlying library supports it).