Hi, you can try this: First open bookmarks > Show All Bookmarks > Import and Backup > Restore, and see if you have a backup there.

If not, please see-

• Profiles - Where Firefox stores your bookmarks, passwords and other user data.

• Recovering important data from an old profile.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

You can check for problems with the places.sqlite database file in the Firefox profile folder.

• 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

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

If you use Sync then best is to disconnect Sync (temporarily) while performing maintenance/repair efforts to the places.sqlite database.