Depends which browser/feature you're talking about. The WebGL stuff I believe is hardware accelerated in Linux Firefox, but some of the video stuff is Windows only for now and some other stuff is Vista & Windows7 only currently.
For the Mac are you talking about H.264 acceleration? Apple have historically not exposed the hardware acceleration features of the graphics cards they shipped, often not even using it themselves and just using software decode. They've changed this recently by making use of some (but not all) of the Nvidia (but not ATI) decode capabilities but it's not the hardware that's missing for many older devices, it's access to the hardware that only Apple can provide. These capabilities are accessible if you're running Windows or Linux on that same hardware.
For the Mac are you talking about H.264 acceleration? Apple have historically not exposed the hardware acceleration features of the graphics cards they shipped, often not even using it themselves and just using software decode. They've changed this recently by making use of some (but not all) of the Nvidia (but not ATI) decode capabilities but it's not the hardware that's missing for many older devices, it's access to the hardware that only Apple can provide. These capabilities are accessible if you're running Windows or Linux on that same hardware.