Settings -> General -> Performance. Turn off both checkboxes (not sure if you need to, but just to be sure...) and restart Firefox.
Bug 1817617 Comment 13 Edit History
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Settings -> General -> Performance. Turn off both checkboxes (not sure if you need to, but just to be sure...) and restart Firefox. When it starts, `about:support` should show that WebRender is in "Software" mode.