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_fix-the-bookmarks-file

You can also try to repair the places database with this extension:

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

Much appreciated! Sorry it took so long for me to get back to this. I ended up deleting the places.sqlite files (following your links) and restarting Firefox, and my bookmarks/functionality have returned. I may have lost my history, but that's far less relevant than my bookmarks.

Thanks so much!