Jellyfin is more like a self hosted streaming service for movies and shows.
It can only watch content you provide yourself on your server, with that content coming from fully legal sources.
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.
It's FOSS, respects my privacy, doesn't try to kill my adblock and it's the only option that doesn't support a big evil monopoly