having a tuner card and direct hooking up your cable or antenna to the pc and using it as a set top box.
streaming media off the internet via netflix, itunes or some other medium.
mozilla firefox is a web browser.... i havent used netflix rentals or other internet tv so i'm not sure if they launch right in a webbrowser or if they have a dedicated program. if you are having trouble with a streaming video rental service or similar media playing in a certain browser then consult the rental company as there are most likely fixes.
describing your issue in greater detail would also help.