See if this add-on meets your needs.
https://addons.mozilla.org/en-US/firefox/addon/show-parent-folder/

Or this one.
https://addons.mozilla.org/en-US/firefox/addon/go-parent-folder/

There's a support thread here - http://forums.mozillazine.org/viewtopic.php?t=659690 - specifically for this add-on.

And for Show Parent Folder the support thread is here - http://forums.mozillazine.org/viewtopic.php?t=659689

Both those support thread are listed as the Support Site on the download page for each respective add-on, on the right of About this Add-on.

Without using one the extensions mentioned above, use this method:

1. Open the bookmark in a tab with a left-click or a middle-click.
2. Click the yellow (blue on a Mac) star on the location bar to open the "Edit This Bookmark" window
3. Click the arrow at the far right of the Folder field to see the folder in which that bookmark is stored.

You can do this in "File > Work Offline" mode to prevent opening the link.

Thanks cor-el, that helps a little bit: If I am able to find a bookmark that is stored in that folder (perhaps by using Firefox's folder search box), your method will allow me to find the name of the folder it is stored in. However, I do not see how that will help me find the other bookmarks stored in that folder, which is the problem I am trying to solve.

Many thanks, Ned

Thanks, ed-meister, but I do not understand how these Add-Ons could help me find a sub-folder buried deep in my hierarchy of bookmark folders and sub-folders.

You can open the full folder tree if you click the arrow at the far right in the edit this bookmark dialog. That should give you an idea where this bookmark is saved.

Note that you can locate (search) a folder via the left pane in the Bookmarks/History Manager (Library) if you select a folder in the left pane and start typing the first letters of the folder name (you need to press the key quick enough to prevent starting a new search).
This also works in the bookmarks sidebar (View > Sidebar) for both bookmarks and folders..

Thanks again, cor-el

   I can see now that once I am editing a bookmark by "clicking the arrow at the far right" Firefox will open up the hierarchy of my folders and sub-folders and highlight in blue the name of the folder where this bookmark is stored.  So, this is a solution IF if can find a bookmark stored in the folder that I am looking for.  (I also see now that the Add-On, https://addons.mozilla.org/en-US/firefox/addon/show-parent-folder/  , will also list the folder where a bookmark is stored in the result listing of a Firefox bookmark search.)   But the problem is that for either of these methods to work, first I need to find a bookmark that is stored in the folder I am looking for.

     With regard to your suggestions of keying the first letters of the folder name in the  the left pane in the Bookmarks/History Manager (Library) or in the bookmarks sidebar (View > Sidebar):   I can indeed find folders this way, but only if their parent folder is open showing the sub-folder I am looking for.   This technique would work if I had some convenient way of opening all of the folders and sub-folders to reveal my entire hierarchy.  But I do not know of such a way.

Again, many thanks, Ned

Shamefully, there does not seem to be any decent Firefox solution/add-on for these very simple features. It is indeed annoying, as I just recently migrated from Opera to Firefox after 10 years of using Opera mainly because its decently developed bookmarks manager UI. In Opera, the bookmarks directory tree (consisting of my thousands of bookmarks) is narrowed down in real-time by typing letters one-by-one into the search box. Simultaneously, all parent folders are getting opened in the tree. And of course, entries are matched not only from the beginning of a word, but from wherever the substring occurs... Plus, the search matches also folder names, not only bookmark names...