As of early this morning
(
https://hg.mozilla.org/mozilla-central/rev/16fa627201d1) we removed
support for disabling WebRender from mozilla-central. This means that
WebRender will always be enabled except for during some APZ gtests.
This is a prerequisite for beginning to remove the code for
non-WebRender code paths which is going to start happening in this
release cycle.
If, for some reason, you need to test with WebRender turned off it's
still possible to do that in beta 92, and will remain possible in
91/ESR.
-Jeff