No, only tried on stable. Looking forward to testing 67 when I get the update.
I should clarify that I am talking about the situation when running Firefox with GDK_BACKEND=wayland.
UPDATE: Ooh! Firefox 67 in my package manager! I am happy to report that I am now able to use Firefox with WebRender on Wayland directly. One glitch so far: It leaves a one-pixel row transparent at the bottom of the screen. Weird :)