Firefox periodically backups your bookmarks. See if you can restore them through Ctrl+Shift+B => Import and Backup => Restore

Apparently it's a rare bug that pops up under specific circumstances for some users. I have not yet been able to reproduce it on my end so unfortunately all I can advise is to export backups of your private bookmarks once in a while so you can restore them if you encounter this again. Thanks for the feedback.

Thank you!