Firefox will happily accept self-signed certificates chaining to manually imported CAs. However, there are a lot of severely outdated guides on creating self-signed certificates out there, and many of the certificates produced that way won't be accepted by any modern browser. OpenSSL's terrible command-line UX certainly doesn't help matters. I've found easypki[1] to be the most convenient tool for this purpose.

[1]: https://github.com/google/easypki

The person you're replying to doesn't have any problem with certs. Their problem is that they (or their employer) hijack a TLD for whatever ludicrous reason, and HSTS pre-loading applies to their hijacked names the same as it would to real names.

Are you sure? The initial post was about .dev being HSTS-preloaded, but the comment I was replying to was an answer to the suggestion that they could use self-signed certificates after importing them to the trust store.

Yeah, and having read over this thread about three times I'm actually less sure than I was. I'll be checking today.

Ah, got it.

Well, another argument in favor of not overloading TLDs for internal domains, then, and just buying an additional domain if you really want to have separate internal and external domains.