Initial load of /latest on Firefox is not compatible with some extensions

Continuing the discussion from RSS "bug" in Firefox?:

That’s 404.

Please, does anyone have a usable (or canonical) URL for the bug?

Background

One-star https://addons.mozilla.org/addon/want-my-rss/reviews/1955253/

This doesn’t appear to work at discuss.kde.org.

– that sparked my curiosity, so then:

– then I made the bug reproducible at Mozilla Discourse.