mesa_glthread=false is only set on legacy X11. You won't get the this irrelevant success message on Wayland (MOZ_ENABLE_WAYLAND=1, bug 1752398).
Bug 1744389 Comment 24 Edit History
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mesa_glthread=false is only set on legacy X11. You won't get this irrelevant success message on Wayland (MOZ_ENABLE_WAYLAND=1, bug 1752398).