That's why for Firefox Relay [1], we recommend using random email masks, and why I use it over my own catch-all domain for unimportant things.
We do support your own catch-all subdomain, but that is just to cover the use case of having to come up with an email mask on-the-spot. If you are filling in a web form, best to use an email address that looks exactly like the email addresses of other Relay users.