I know about two times you could expect to see that: (1) restricted sites like Mozilla sites, and (2) sites that protect themselves from extension scripts using the "sandbox" security policy (#2 was fixed in Firefox 128).

Do you want to share some example URLs by sending them to jscher2000@outlook.com or posting in this thread: https://github.com/jscher2000/Save-webP-as-extension/issues/30