Firefox 84 Beta, activates WebRender by default on Linux. Mozilla has gradually enabled WebRender by default in Windows and macOS under various versions based on GPU / driver configurations.
Linux users can run Firefox by activating:
MOZ_WEBRENDER = 1
to force WebRender activation.
Starting with Firefox 84, it will be active by default in certain Linux configurations (initially, WebRender on Linux will only be active if you run Firefox from the GNOME and X.Org/X11 desktop environment)