Which bookmark did you remove?

Is that one on the smart bookmarks folders that show a list of bookmarks stored elsewhere or a real bookmark?

Note that if you have accidentally removed bookmarks you can use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to get them back.
The Organize button is the first of the three buttons next to the Back and Forward buttons on the toolbar in the Library (Bookmarks Manager).
That only works if you haven't closed Firefox.
If it didn't work then you can restore the bookmarks from a JSON backup.
This will replace all current bookmarks and you lose bookmarks that were added since that backup was created.

See:

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

Thanks cor-el. I cannot find, anywhere in Bookmarks, an 'undo' button/option. (I don't know what the difference between a 'smart' and 'real' bookmark. It was real enough, though maybe not so smart as it got deleted).

There isn't a third button, and I access 'Organize' via the 'Bookmarks' drop-down menu.

You should have a back and forward button and three other buttons on the toolbar in the Bookmarks/History Manager (Bookmarks > Organize Bookmarks).

You will probably have to restore the bookmarks from a JSON backup.
This will replace all the current bookmarks with the ones in the backup.

• "Most Visited" and "Recently Bookmarked" and "Recent Tags" are examples of so called Smart Bookmarks folders and are not real existing folders.
• Such smart folders show a list created by a query of the places.sqlite database that stores the bookmarks and the history in Firefox.

• https://support.mozilla.org/kb/Restore+the+default+Smart+Bookmarks+Folders