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.
That does not work because the browser that does not know how much it can reduce the presented feature-set without actually breaking the site. Some content is vpx-only.