(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…
I can't say I agree. That would be a good idea in a world where users never encountered errors in normal circumstances, but in fact computers don't work very well and users encounter errors on a regular basis. Using computers is a skill. It takes study. You can either pretend that it doesn't, giving users something underpowered that is going to break anyway, or acknowledge that computers are complicated, and build tools and resources to help users navigate them. And users should recognize it as folly to try to use the most complicated tools on the planet today without any understanding of how they work.
This doesn't really apply to HTTP status codes. They're largely useless. It does apply to the way my Macintosh told me of a misconfiguration after I reinstalled the operating system. Somehow the people at Apple thought an error code and a brief description would be scary, so they opted for this absolutely terrifying icon instead:
https://i.stack.imgur.com/Y1teY.jpg
(Not my picture, by the way.)