Firefox automatically creates 5 backup copies of the bookmarks, one for each of the last 5 days you used Firefox. For details of how to restore a backup fiile see restoring from backups.

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