Command line arguments will disable skeleton UI except a few approved one: https://searchfox.org/mozilla-central/rev/b9d9a1a873302452fc918505d62cd226d81b67d0/mozglue/misc/PreXULSkeletonUI.cpp#1638-1660 You will have to launch Firefox without any command line argument to test this bug.
Bug 1824110 Comment 10 Edit History
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Command line arguments will disable skeleton UI except a few approved one: https://searchfox.org/mozilla-central/rev/b9d9a1a873302452fc918505d62cd226d81b67d0/mozglue/misc/PreXULSkeletonUI.cpp#1638-1660 You will have to launch Firefox without any command line arguments to test this bug.