That wasn't new for Firefox 20, it's been like that for a hell of a lot longer - like maybe Firefox 3.0 or 3.6.

Two ways to get beyond that limitation.
1. Save a new bookmark, then open the Library window and make a copy of that bookmark.

2. Open the Bookmarks Sidebar and drag the website icon from the left corner of a tab down into the Sidebar into the folder where you want the duplicate saved. Assuming you already have a bookmark saved and you are adding a duplicate - although this works even if it isn't saved.

See also:

• Old Add Bookmark Behavior: https://addons.mozilla.org/firefox/addon/old-add-bookmark-behavior/