The screen casting feature is an experimental feature, and for now, only Chrome can stream the desktop. Other browsers (for example, Firefox) can receive the screen cast data streamed by Chrome and show it, but they don't support screen casting itself as yet.
Streaming of prerecorded media can work differently in different browsers:
Firefox will start playing the streamed data right after it has received the first chunk
Chrome will wait until all the chunks are received (the whole media file) and will start playing only after that