The main change in Firefox is to use the system compositor (via CoreAnimation) to composite components of Firefox windows. Until just recently, Firefox composited its entire window into a buffer using OpenGL, and then sent that to the system compositor. Unfortunately, with that method, you have to send the entire window to the system compositor on every frame (because the APIs are just limited that way), which uses a lot of power when just one little caret is blinking, for example. With CoreAnimation you can set things up so that large chunks of the window that aren't changing never get recomposited.
Unfortunately this change is somewhat invasive, which is why it wasn't done long ago.
Unfortunately this change is somewhat invasive, which is why it wasn't done long ago.