Normally, right-click > Delete Bookmark will take care of it.
It's possible that Sync is detecting a difference between the list of bookmarks in the browser and the list in the cloud, and restoring the deleted bookmark. I don't know how to solve that, so I suggest posting that on the Mozilla Support site. Here's a link to the new question form (for Windows/Mac/Linux): https://mzl.la/3y1vQUQ
Another possibility is that bookmarks are broken due to a file access or corruption issue. If you cannot add a new bookmark, try a regular shutdown and restart of your computer to release file locks. If that doesn't help, you can try the "Verify Integrity" routine on the Troubleshooting Information page (first link below). And if that also fails, see the second link below.
