If you have accidentally removed bookmarks then use "Organize > Undo" in the bookmarks manager (Bookmarks > Organize Bookmarks) to get them back.
The Organize button is the first of the three 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

This happened yesterday and I turned off my computer last night. Method one will not help, unfortunately

All I lost was one MAJOR folder. I still have many other bookmarks which I do not want to lose. If I use the joson backup idea, will I lose the many bookmarks which I still have?

If you restore bookmarks from a JSON backup then you lose the bookmarks that have been added after that backup has been created. You can export the current bookmarks to an HTML file and import that HTML file after you have restored the JSON backup.

See this extension on how to remove duplicate bookmarks.

• CheckPlaces: https://addons.mozilla.org/en-US/firefox/addon/checkplaces/