Got some screenshots on this private reddit post.
I can tell it's the link icon for the Unified Settings advertised at the bottom of the page. I started FF in safe mode, problem went away, then enabled plugins one by one and now I can re-create the issue if I toggle noscript in add-ons menu.
All JS is enabled when this happens, sometimes it seems to disappear after temporarily enabling global JS, but after a refresh it's back. The ctrl+shift+j console shows only one line on a newly refreshed page with the error.
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