I kicked the tyres on the new Quantum'd Firefox. Oddly I found that the GPU utilisation on my MacBook Pro is significantly higher than Chrome, even higher than Safari. Playing a YouTube video would make the MBP's fans spin significantly faster than normal which meant it' wasn't overly usable. The numbers were roughly 750 Mhz Firefox, 550 Mhz Chrome and 450 Mhz with Safari.

Not sure if it's an option I've got configured, I'd love to find out what was causing this though!