Are you using sync? Go to the [https://addons.mozilla.org/en-US/firefox/ Mozilla Add-ons Web Page] {web Link} (There’s a lot of good stuff here) and search for a duplicate bookmark checker.

If you use Sync then best is to disconnect Sync temporarily during troubleshooting. This can be caused by a problem with the <b>places.sqlite</b> and <b>favicons.sqlite</b> databases in the Firefox profile folder. *use "Verify Integrity" button on the "Help -> 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 a 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 See also: *https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file *https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page. *Help -> 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