Firefox automatically creates bookmark backups.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

In what directory?

In the user Profile folder.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

If it happens again:

If you have accidentally removed bookmarks then 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 in the bookmarkbackups folder.

• Bookmarks > Show All Bookmarks > Import & Backup > Restore

This will replace all current bookmarks and you lose bookmarks that were added since that backup was created.

See also:

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