Firefox has really been appealing to me lately. Unfortunately, on my MacBook Pro, Safari is more energy efficient and it's not even close. If Firefox could close the gap there a bit I would make the switch.
That said Mozilla is apparently working on it and to improve energy efficiency, try setting gfx.compositor.glcontext.opaque to true in about:config.
Courtesy of @pcwalton: https://news.ycombinator.com/item?id=18048844