Have you tried using the Bookmarks Sidebar? The Bookmarks Sidebar will retain its position as long as it is not closed and re-opened; in which case it will "rewind" to the top of the list. I too have a hierarchical nested folder arrangement with thousands of bookmarks and use the Sidebar when working intensely with bookmarks.

• Open or close the Bookmarks Sidebar with CTRL+B

You could also keep the Library window open to that folder, then click on the Library window in the Windows task bar at the bottom of the screen to bring that window to the top to make another selection from the folder. In the Library window, click on the desired folder, the bookmarks within that folder appear in the right panel and it will stay put until you change folders in the left panel of the Library window.

• Open the Library window with Bookmakrs > Show All Bookmarks or CTRL+SHIFT+B

Take a look at this add-on.
https://addons.mozilla.org/en-US/firefox/addon/bookmarks-history-panel/?src=ss

There's a "pin" to hold the dropdown open while it is being used.

TheOldFox: As I mentioned before, I don't want to eat up screen space by displaying anything like the sidebar or toolbar. I'd like to limit the bookmark menu to a button with a dropdown.

I think I've found an add-on that does mimic the bookmark button and is sticky. I'll try that out.

Thanks for your input.

The ed-meister: Thanks for pointing me to the add-on. It's pretty much what I was looking for.