The new Places-based Bookmark Manager includes back/forward buttons that appear to the left of the search box (in the menu bar): The back button appears once you have switched to another folder than the default, and the forward button appears once you have used the back button. Both can only be found by hovering over the area next to the search box since both are missing icons. Firefox has icons for both buttons and shows them at the very left of its Library main toolbar. Only checked with Win7 and the default theme, but other OSs and Modern are probably affected, too.
Created attachment 489947 [details] Screenshot of Firefox Library toolbar This is how the firefox toolbar looks on Mac
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