Are you using a "scaled" resolution? If so, there's a bug in redrawing that pegs a core, and kills battery life.

It can be mitigated by setting

  gfx.compositor.glcontext.opaque = true

in about:config, but the fix is invasive, and has taken years to get prioritized and worked on. I've seen reports that it should finally be done in FF64 or 65.