OK, so... I right clicked, saved-as and then ran the XPI... and that worked. So thank you for that suggestion.
As for the studies image, that's only half of the fix according to the blog post [1], mine is only verification-timestamp, not signing-intermediate-bug.
As for the studies image, that's only half of the fix according to the blog post [1], mine is only verification-timestamp, not signing-intermediate-bug.
[1]: https://blog.mozilla.org/addons/2019/05/04/update-regarding-...