Could be caused by using Sync to sync bookmarks to a mobile device.

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

You can check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• use "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page
  in case errors are reported close and restart Firefox and retry

See also:

• http://kb.mozillazine.org/Locked_or_damaged_places.sqlite
• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

If "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page cannot repair places.sqlite then remove all places.sqlite files in the Firefox profile folder. 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
• http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

The deletion of the plac.sqlite didn't help

You may have to restore an older backup from the bookmarkbackups folder if the most recent backup that Firefox has used to rebuild the bookmarks is already affected.

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore

The actual bookmarks are ok. I restored from a month ago but the problem persists. The problem is that the list is duplicated

Try to use this extension.

This looks promising. I loaded the extension but I cant get to the next stage. Clicking on the organge/green sats does nothing and selecting "options" just brings up the text which starts as can be seen in the attachment. Can you help me here?

Keith

Sorry you are having that problem. Be aware that Restore of bookmarkbackups files will overwrite the existing bookmarks in Firefox, but if you don't select an older bookmarkbackup from before the corruption actually happened to that Firefox installation via the Sync feature you just be "restoring" the corruption! Typically ALL bookmarks may be duplicated if it isn't caught real fast.

Plus with the Import feature in Firefox Bookmarks, any already existing bookmarks on that device WILL be duplicated due to the Import feature appending existing by design (IOW supposed to happen). Typically it is "here and there" with the imported bookmarks with not specific discernible pattern to the duplication.

I mention both because I haven't seen the double-line before separating the duplicated groups before from other screenshots that users have posted in the past. But is rare for user with that problem to even post a screenshot that shows that so clearly as your screenshot does, I am left wondering if that "feature" is something new in Quantum that I never payed attention to before or if it is just new to Quantum.

Thanks for the reply. I am able to delete all the duplicates so that I have a "normal" list of bookmarks. However, the lower list is still there just empty. All very interesting