Sync doesn't duplicate bookmarks, but using Import Bookmarks from HTML ... will append existing bookmarks. So depending upon which set of bookmarks that you imported you could get duplicates or completely duplicated bookmarks.

But if you deleted what looked like duplicates, my guess is that the place.sqlite file was corrupt which "we" have seen reports of from users of Sync when they were sync'ing a Desktop Firefox with a "Mobile" Firefox; either Firefox for iOS or Firefox for Android.

Help > Troubleshooting Information --> near the bottom of that display see Places Database to "Verify Integrity" of the database.

But that won't restore the bookmarks that you inadvertently deleted.

I think you'll need to restore the last known good backup of your bookmarks, probably from the day before they were deleted or before they were corrupted with duplicates. See this support article: https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them#w_restoring-from-backups

Where is help? Sorry, feeling dumb but can't find it. This was a completely new Firefox installation. There were no bookmarks except what Firefox includes. I was syncing from win 7 desktop to an HP G71 notebook PC. I have only just installed Ubuntu, and deleted Win 7. I'd like to get rid of all the bookmarks and start from scratch but I can't find any way to do that except one at a time that will take forever

Sorry if my answer caused confusion.

Two things could have created the duplicated bookmarks condition.

1. Sync and/or Firefox could have been corrupt and caused what you saw. The first step to fixing that is to verify the integrity of the Places database; preferably with Firefox disconnected from Sync or your internet connection disabled. Which seems the most likely cause, seeing that deleting one group of duplicates caused the 2nd group to disappear.

• Did you "Verify Integrity" of the Places Database inside Firefox?
• Or are you having trouble following my instructions about how to do that?

2. Using Sync and then Importing a bookmarks.html file will definitely cause duplicated bookmarks. The "Import" feature adds to any existing bookmarks.

• Does that HP G71 notebook PC also have duplicated bookmarks?

The Sync process may have caused the bookmarks in that Firefox installation become duplicated, too.

If so, you probably have a good set of bookmarks on that device from before the Sync process transferred the corruption to that device.

• After reading the support article that I linked, did you locate the bookmarkbackups folder and view the 15 backup files in that folder?