Your first 5 email masks are free and if you install the extension a little icon will appear in most email fields. Let’s you create a new mask right there.
If you buy the premium version you can get your own custom subdomain: @XXXX.mozmail.com where you pick XXXX
This way you don’t even need the extension. You can just do something like “[email protected]” and Relay will “create” that email for you. Cannot recommend it enough, especially since it’s free to start