There used to an firefox addon called Old Add Bookmark Behavior which allows creating duplicate bookmarks but now stops working. Allowing creating duplicate bookmarks is a very important function but now missing from Firefox. Who will kindly revive this addon or this function? Thanks!
I didn’t realize that you can duplicate bookmarks! I was doing it the hard way by clicking on the “Bookmark this page” icon and placing it in a second location.
It turns out that in both Firefox 55.0.3 and Firefox Nightly 57.0a1 (2017-08-25) (64-bit) duplicating bookmarks is quite easy!
When one has the Bookmarks Library open (e.g., one of the links to “Show all Bookmarks” or Ctrl+Shift+B in Windows), one can Right+Click on a bookmark or on a folder, click “Copy” from the context menu, navigate to where one ones the copy and on the right half of the library screen Right+Click and pick “Paste” from the context menu. The bookmark or the folder of bookmarks (with its containing bookmarks) is copied.
The same can be done if one has a tree of bookmarks being displayed. On the Bookmark Toolbar, for example, the first “Bookmark” I have is actually a folder structure containing all my bookmarks that aren’t on the Bookmark Toolbar and, once that folder is expanded, I can copy and paste and rearrange like I can in the Bookmarks Library, except that the only place I found I can insert separators are in the Bookmarks Library.
If paired with some extension like Bookmarks Organizer for finding duplicates, dead bookmarks, bookmarks that go to immediate redirections and suggest replacement with the landing URL, this can be quite handy. (Warning: if using “Bookmarks Organizer” to find duplicates, do not allow it to automatically delete duplicates. Actually, I don’t allow it to make any changes automatically.) In my case, I keep “Bookmarks Organizer” disabled except for when I want to do bookmark housecleaning. And since it is pure WebExtensions, it will even work in Firefox 57 and beyond. 
If you are looking for some other functionality, you may have to describe the specific function(s) you are looking for; someone may have an idea if you want something that isn’t provided as part of the Firefox user interface.
I am curious: anyone out there knows how I can get a shortcut for those things mentioned above: “Show all bookmarks” or “Ctrl+Shift+B” i.e. getting them all shown in one click. There’s gotta be an alternative to this going to bookmarks AND show them all (or that other shortcut), I just can’t find it … 