(I'm not sure there's any way to find the HTTP status code in a modern Firefox environment short of using web developer tools. It's not in places like 'Page Info' as far as I can see.) I'm old enough to remember times before browsers showed 'friendly' error messages, and a 500 response from a server would display a '500 Server Error' message to the user. Users hated it. They'd panic and think they'd broken the websit…
When did we stop building software for the "Power User" persona? I don't disagree that there are some (many?) people who start screaming and running in circles when they see an error code, but not every user is like that. You don't have to be a dev to care any the distinction between a system doing its job and silently failing. Why not have some indication that there's smoke coming out from the engine?
I think we just have fight for poweruser options to be available via APIs and support the add-on ecosystem.