I remember once reading here that there was a bug that made Windows show up as “unknown”

If I remember correctly if you use your browser’s password manager anyone with access to your PC can see your passwords, with Bitwarden or Keepass (or any other password manager) you need the master password.

Also I thought that the browser also saves the passwords unencrypted but I’m not sure if that’s true.

I’m prettey sure Firefox can do that, and like others said: it is better to use a password manager like Bitwarden (you can also selfhost it) instead of Firefox’s built in one.