You can use [1] to force YouTube videos being streamed as H.264 instead of VP8/9. This helps with the battery issue (at least on YouTube).
[1]: https://addons.mozilla.org/en-US/firefox/addon/h264ify/
[1]: https://addons.mozilla.org/en-US/firefox/addon/h264ify/
Just because Firefox technically supports a format doesn't mean it has to disclose that format to external servers for selection.