Registering for what service? A Firefox account? We might not be able to help you unless it is a Mozilla-related service. Do you get any more error messages than "Please enter a valid email address"? Anyway, make sure that the address is entered correctly (space pasted before or after is a common mistake).

This service. I've used it before now,but I don't think that I ever registered.

Mozilla is divided into several separate support/service areas. Each uses their own username and password logins.

What are you trying to access?