Hi, sorry to hear you are having a problem; please see if anything here helps - How to fix preferences that won't save.

Note: If you want to try Refresh, be aware that it can very occasionally cause issues of its own, so to be on the safe side, before you do it, I would backup your bookmarks: Open bookmarks > Show All Bookmarks > Import and Backup > Backup. Or, Export Bookmarks to HTML, and save them to somewhere convenient on your computer. Note that Backup will overwrite any existing bookmarks when you restore them. For extra safety, I'd suggest making both a Backup - which creates a JSON file, and an HTML file. You can instead backup all your data by copying your Profile, but this requires some technical expertise - Back up and restore information in Firefox profiles.

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

When does this happen?

Always when you close and restart Firefox or only when you reboot the computer?