A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

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

• http://kb.mozillazine.org/Lost_bookmarks

This turned out to be the root cause for my problem. Simply renamed the places.sqlite.corrupt file to places.sqlite and my profile (including bookmarks) worked again. Thanks!

By the way, I first noticed that something was wrong with my _profile_ when, in addition to lost bookmarks, I noticed that I couldn't create _new_ bookmarks. Also was not able to restore bookmarks from a backup .json file. So I created a new profile, and was able to restore the bookmarks from the original profile's backup. That led me to exploring your suggestion. Thanks again!