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 “Lemmy@XXXX.mozmail.com” and Relay will “create” that email for you. Cannot recommend it enough, especially since it’s free to start

Firefox Relay is by far the easiest (and imo best) solution for that

You can try it for free and if you use it enough it only costs $24 a year