With regard to Firefox, you can use Bookmarks Manager (CTRL+SHIFT+B) to manipulate your existing bookmarks into whatever folder structure you'd like.  It behaves not unlike File Explorer in that you can cut and paste, create new folders, etc., for your bookmarks.   I cannot say how your current folder structure is arranged but it sounds like your old bookmarks are in a subfolder and your new ones at the top level (or perhaps vice versa).

With regard to using Edge, in Settings, there is a section named Import favorites and other info with the button Import from another browser.  If you activate it you will find you can import directly without having to export from the other browser or you can choose to go the old school method of exporting to an HTML file and importing from that file.
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Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134  

    A little kindness from person to person is better than a vast love for all humankind.

           ~ Richard Dehmel