I'm no expert but logically my first hope is that it's part of a Sync process that has gone astray. I DON'T Sync but recently found in the Toolbar Bookmarks 2 Sets of the same bookmarks - one under the other - , and I just manually deleted with no repeat as you have.

If you use Sync, best is to disconnect Sync temporarily during troubleshooting. This could be a problem with the <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder. *use the "Places Database" -> "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry. If "Verify Integrity" cannot repair places.sqlite, rename/remove all <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder with Firefox closed. Firefox will rebuild places.sqlite and restore the bookmarks from the most recent JSON backup in the bookmarkbackups folder. *keep a backup copy of places.sqlite in case a new places.sqlite database has to be created *you lose the history when a new places.sqlite is created You may have to restore an older bookmarks backup in the bookmarkbackups folder via the Bookmarks Manager (Library). *Bookmarks -> Manage Bookmarks -> Import & Backup -> Restore *https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them See also: *https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional *https://support.mozilla.org/en-US/kb/cant-add-change-or-save-bookmarks You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data