Hey, Screenshots person here...this is a new one to me.
My only thoughts would be the pref flip that seems to be ruled out at this point, or that you're trying to shoot an about:[SOMETHING] page for which we disable screenshots.
Firefox has a screenshot feature that I'd like to trigger from Selenium, which is by going through the "..." icon in the URL bar which opens up a menu with its last item being "Take a screenshot" (and then you can select a full page screenshot).
Is there a way to programmatically trigger this feature or sequence?
My only thoughts would be the pref flip that seems to be ruled out at this point, or that you're trying to shoot an about:[SOMETHING] page for which we disable screenshots.
Otherwise, file an issue: https://github.com/mozilla-services/screenshots/issues
We're triaging in an hour or so :)