It will just come back. What many of us do is; Move any unwanted bookmarks (created by Firefox) and move them to the bottom of the bookmarks list.

I don't have the bookmark so it must be possible to delete it. To move it, open Manage Bookmarks (in the menubar menu or use ctl+shft+o), select the bookmark in the right panel and drag it to the folder where you want it (such as Other Bookmarks).

No joy. I can open Manage Bookmarks, grab it with a left-click and (apparently) drag it to the Other Bookmarks list, but nothing changes. It's still sitting stubbornly at the left edge of the Bookmarks Toolbar.

There is a thread from 6 years ago where the user succeeded in deleting the bookmark after searching in bookmarks. https://support.mozilla.org/en-US/questions/1055446

Another possibility is to move it to a folder on the Bookmarks Toolbar.

I checked the thread. That user's problem was that he couldn't see the link on the Bookmarks Menu to delete it. I can see it but it doesn't go away when I right-click on it and select delete or cut. I tried moving it but I can't drag it anywhere. Thanks for the suggestion.

Works for me if I right-click this button on the Bookmarks Toolbar and click "Delete Bookmark".

I checked the thread. That user's problem was that he couldn't see the link on the Bookmarks Menu to delete it. I can see it but it doesn't go away when I right-click on it and select delete or cut. I tried moving it but I can't drag it anywhere. Thanks for the suggestion.

If you can't delete it otherwise, why not try that. Sillier things have worked.

Try what? I'm afraid I don't understand.

Try searching for the bookmark, then selecting, right-clicking the result and deleting. It may delete as a search result if it won't delete otherwise.