This is likely related to the issue I described here:
The following might have happened:
- In earlier versions of Firefox, favicons worked differently - the browser remembered every favicon it saw, thus remembering the “error” favicon and setting this as the bookmark
- Nowadays, Firefox can’t load a newer favicon due to the issue shown in the linked post. Therefore, Firefox uses the last cached favicon from months/years ago - which is the error favicon in your case.
The “bookmark favicon” and the “tab favicon” are two different favicons in Firefox logic, and the bookmark one is broken with syncthing in more recent versions.