The Library is a neglected part of Firefox that very clearly combines decades-old technology (XUL) with new design specs and icons. Unfortunately, it therefore looks like a frankensteined-together part of Firefox that could really, really use a facelift -- and worst of all, it catches you by surprise with its window-based nature when you try to manage your history or bookmarks.
This vital piece of Firefox should be rewritten to be part of a tab, like every other part of Firefox. Modernize it! 🙂