For now, on a retina Mac, you can pick up significant power savings by having Firefox opt out of the OS wide window scaling when on a retina display.
Get info on the Firefox app and then check ”Open in Low Resolution” on the get info panel, then restart the app.
You can pick up more energy savings by telling Firefox to stop using transparent pixels.
Go to about:config, and set gfx.compositor.glcontext.opaque to true.
Firefox has just been overloading the GPU by painting the entire window over and over and sending the output to the window compositor where it is slowed down even more by scaling and dealing with transparency.