As you open the Bookmarks menu the favicons flicker into view.
Do you still see this? See https://bugzilla.mozilla.org/show_bug.cgi?id=1393313#c7
I see this; I'm assuming it's because the icons are loaded on-demand, when the bookmarks menu (and submenus) are first opened. (Not sure it's something that needs to be fixed, though.)
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