I'm on an old laptop with a CPU graph on my top panel. It appears that Firefox uses more CPU than chromium and so I use Chromium when in battery power or when I forget my power cable!
The other thing I dislike with new Firefox is the removal of the ability to change the size of the tabs and alter the window decorations so that they still take up valuable screen estate on smaller netbooks. Chromium isn't any better but does have the option to remove the window title bar.
According to : https://www.phoronix.com/scan.php?page=article&item=firefox-... firefox still has some work to do....