Let's start with this; If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Type https://accounts.firefox.com/settings<enter> in the address bar. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

If any errors are reported: https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

The places.sqlite file contains your History and Bookmarks. And it looks like it is corrupted.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox. The browser should then create a new place.sqlite and recover your bookmarks from the backup files.

Note: Your history is not backed up and will be lost.

Sync is supposed to share data between different profiles and/or systems. But sometimes something goes wrong.

Read these instructions carefully before doing anything ! !

After shutting down sync, I suggest you fix one of the bookmark files. Do Not reconnect sync until you are told.

Once you have fixed one of the profiles,

Shut down the old sync.

Delete Account: Look for Delete Account at the bottom of https://accounts.firefox.com/settings

This will clean out the old data

After, create a new sync using the system/profile that has the data. Wait for a while until the syncing is done before adding any new members to the sync.

Note: Before adding any other members, open them in the browser and remove its bookmarks.