In addition to battery life, I'd also like things like native PiP mode (if possible) and native, fullscreen video playback. Better rendering gets closer to letting Firefox replace Safari for me, but the other features are important to me, too.
Native PiP is coming in Firefox 71 (already available for testing on Beta releases), though Windows-only at first. You can track macOS progress by following:
https://bugzilla.mozilla.org/show_bug.cgi?id=1532675
Picture-in-picture is possible, but it relies on SPI on macOS (for which headers happen to be in the WebKit repository, so there are third-party applications that use it already).
Native PiP and fullscreen video playback are both available in developer edition on Windows for me and the former has been around for at least a month. I've had fullscreen video playback for years so I'm not sure what the deal is with that one...
Hopefully mac users get both.