This is a really cool concept - like really cool - but I feel that it's not being executed very effectively. Firstly, the little icon that appears in the 'email' fields of sign-ups doesn't allow you to click on the icon, select the email you want to fill it with, and then actually fill the email field. It wants you to re-direct to the actual page, tab over, copy it, then paste it into the email field. Horrible? No. Easy? No. Not too much, but steps can certainly be reduced for better user integration/convenience.
Secondly, MANY websites will not accept these 'mozmail.com' email addresses into their databases. Many will tell you that they aren't valid addresses. I'm not sure if this is an error on the user-end xp, or if it's an actual attempt to halt Firefox from being able to re-route marketing traffic. Either way, this is something that the dev team should certainly look into. I think this has awesome potential, but there are a few tweaks that could make this a mastapeace. (misspelled intentionally).