I'm curious, if you go to about:support are HW_COMPOSITING and OPENGL_COMPOSITING enabled?

Someone (likely here) mentioned that setting layers.acceleration.force-enabled to true in about:config makes a dramatic difference in performance, and based on my limited experience, it does. So much so, that I'm thinking about switching to Firefox after many years of only using it sparingly due to lousy performance on Linux compared to Chrome (e10s has helped too).

edit: I should note that HW_COMPOSITING and OPENGL_COMPOSITING are not enabled by default on the 2 Arch Linux systems I have (Ivy Bridge and Haswell iGPUs, up to date Firefox, Mesa and Kernel). Not sure if there is a white/blacklist, or if its just always disabled in Linux.