I tried it on my Rift in Firefox, and fired up the HUD using OTT to see the framerates. Mine were usually <45, occasionally in places jumping higher but also dropping down to 0.
Changing the Firefox performance option to use the maximum available processes:
Improved the result immensely, providing a much smoother experience and generally higher framerate, usually not <45. Also tried it with ASW off, which may have had a small effect.
I don't have the best VR performance machine, so others probably will have even more positive results.