Are you using sync? Shut it down for all devices until the problem is gone. Once the problem is fixed, do the same on all systems before running sync.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

Yes, I am running sync. I will turn it off and see what happens,

I am trying this solution.

Any chance you are using Sync with either Firefox for Android or Firefox for iOS, along with Laptop / Desktop computers?

If so, I strongly suspect that corruption of your bookmarks might be related to "mobile" Firefox. My thoughts are that the different separate Bookmarks Toolbar folders used on "Mobile" devices is corrupting data on the Sync server and causing duplication of bookmarks on the Laptop / Desktop versions of Firefox. Did you check for duplicates in the Bookmarks Menu folder or duplicates of the Unsorted Bookmarks folder?

Regardless of whether a "Mobile" device is on that Sync account or not ... When going to fix that "duplication" problem, make sure that you reset your Firefox Account / Sync password before re-connecting any device to Sync. https://accounts.firefox.com/reset_password That will delete the data which is on the Sync server, giving you a fresh start with Sync; thus dumping the corrupt data there.

Use the Places Maintenance extension to "clean" the places.sqlite file where bookmarks & browsing history are stored. That should fis the corruption on the device it is used on. https://addons.mozilla.org/en-US/firefox/addon/places-maintenance/

And if that corruption has propagated to your other devices run Places Maintenance on those devices, too. If you skip either the Sync password reset or skip one device, the corruption may reoccur.