Do you use Firefox's Sync or other data mirroring software? This could be a data synchronisation or file version error (old file was loaded by mistake).

The easiest method would be using the browser's automatic backups: https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them#w_restoring-from-backups. Make backups of your current bookmarks before proceeding to try out old backups.

After that it's down to manual backups, as with any form of electronic information. You can try to pull down a new set from Sync or whatever synchronisation software you use, but that's a hail mary at this point, as chances are the information now matches what you see copied locally.

If you can reverse in file versions, places.sqlite (https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data#w_what-information-is-stored-in-my-profile) is the file that contains your bookmarks.

PS. The help article, https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks#w_no-bookmarks-are-visible-after-installing-an-add-on, points out that improper closing of Firefox may affect bookmarks. The article recommends rebooting your computer - I would, again, make backups of what you have left before trying the reboot.

You can check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• https://addons.mozilla.org/firefox/addon/places-maintenance/

See also:

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

• https://support.mozilla.org/kb/restore-bookmarks-from-backup-or-move-them
• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox