It seems that it's true for the following platforms: XP_WIN || MOZ_WIDGET_GTK || XP_MACOSX: https://searchfox.org/mozilla-central/source/toolkit/xre/nsAppRunner.cpp#3922
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It seems that it's true for the following platforms: XP_WIN || MOZ_WIDGET_GTK || XP_MACOSX: https://searchfox.org/mozilla-central/source/toolkit/xre/nsAppRunner.cpp#3922 On other systems we will not see that error, but according to gfxPlatform::IsHeadless()), the mode is headless indeed.