It's not in the same category. You can use Firefox Relay as an alias generator within Bitwarden. It provides a convenient UI and an integration with the password manager.

I still don't understand why I would need Bitwarden if I use Firefox Relay, sorry.

Integration. When signing up with a new web service, you can just pop open Bitwarden and will generate both a unique email alias and a unique password, prefill the sign up form, and save the details to the password manager.

If I'm not mistaking, Firefox Sync will do that too.