As of Firefox 92, WebRender has shipped to our entire release population. In 92, the pref that allowed fallback to layers was reset. In 93 it was removed.
WebRender - RESOLVED FIXED
Shipping to all versions of MacOS (Firefox 83), including Big Sur (Firefox 84)
Shipping to all versions of Windows (Firefox 92).
Shipping to all devices (Firefox 91). Also shipping to all desktops except Wayland.
Shipping to all devices (Firefox 92).