I added an issue to the extension's github, the plot thickens: https://github.com/jscher2000/Save-webP-as-extension/issues/30
Hmm, the extension tries to inject the script that generates the button bar, but it fails with this error (in the add-on debugging inspector console):
Uncaught (in promise) Error: Missing host permission for the tab
I have not seen this on other images displayed in Firefox's stand-alone image viewer.
On the regular dev tools network tab for the image, I see this that the image is served with this header:
content-security-policy: script-src 'none'; frame-src 'none'; sandbox;
That seems to block even the built-in Firefox Screenshot feature (which uses extension APIs).
I wonder whether applying the site's CSP restrictions to the stand-alone image viewer was intentional or is a new bug. Have you used the extension on this site in earlier versions of Firefox?
Anyway, I'll have to think about whether there is a workaround for this. (Many extensions manipulate CSP for content blocking, and I don't want to interfere with or break those.) And if there isn't, how to at least alert the user to the nature of the failure.
Pretty sure I saw clips of this guy, he very clearly was going out looking to antagnonise people. This isn't America.