I think browser makers (or whoever it needs to be) should offer an option to save battery over a little bit of extra bandwidth. Maybe even make it the default.
And what vim_coder and ConputerGuru meant was that Firefox would present a spoofed list of supported codecs to YouTube to force selection of the higher bandwidth, lower-powered codec.
Just because Firefox technically supports a format doesn't mean it has to disclose that format to external servers for selection.