There is a roundabout way: https://support.mozilla.org/en-US/questions/1161879 "Search bookmarks" in the top right of the Library window (ctl+shft+o) will find the bookmark for you so you can open it (but not tell you which folder contains it).

If this is a bookmark that opens a webpage then you can open this bookmark in a tab. If you click the highlighted star in the location then you can check the location via the folder line. If you click the second arrow at the far right of the folder line then you can drag the scroll bar to see what folder is selected. If the star isn't highlighted then try this in "Work Offline" mode.

See also this extension.

• Bookmark search plus 2 https://addons.mozilla.org/firefox/addon/bookmark-search-plus-2/

Why does the Firefox Bookmarks search results show multiple identical links that appear to be in the same folder, since the specified folder can only contain one copy of an identical link? I searched bookmarks and the search results contains multiple identical entries. When I use the suggested method of actually opening the web page and pretending to want to edit the bookmark to see where it is, then hoping to escape without inadvertently moving the link since there is no Cancel button on the editor, all of the duplicate entries are shown to be in the same folder. I would like to delete the evidently inappropriately duplicated entries but they supposedly exist in the same folder. Why does the search report one link several times in the search results list? This make organizing the Bookmarks folders almost impossible. I found another question (posted by Louie in January 2020) about this same problem that says it had been archived a year ago. The question should not have been archived. Instead, the problem should have been addressed. Please do not archive this thread until a better solution is found.

This is a P.S. to a reply I just submitted. I looked at the suggested Firefox extension for searching the bookmarks folder (Bookmark Search Plus 2), but there is a notation indicating that the extension is not monitored for security and it is on me to decide whether or not to trust it. I looked at the reviews and decided that I cannot trust it.

If you open a bookmark in a tab and click the star then you will always see the first found presence marked in the folder line and Remove gets a number to indicate that there are more copies. You would have to use an extension that shows each of the copies or possibly search for the bookmark in the Library and make all copies distinct by adding a GET parameter to the URL to make it easier to locate each copy.