A service like this is necessary, so the idea is good although certainly not new: there are already dozens of temp email sites, and this is no better. The domain name gives it away immediately, so it's a matter of days for the spammers to add relay.firefox.com to the list of "invalid" email address they won't accept for signing up.

What we really need is temp addresses with a legit domain! For example, @mozillamail.com would be fine, as long as it provides real addresses too - just like, say, gmail. What matters is that the same domain hosts both temp and legit addresses, so the spammers can't tell temporary from legitimate. Yes, it's probably harder on the server. But short of that, such a service is nearly useless, as is the extension.

If/when done like this, users will want an easy way to reply to the spammer from the legit address via the temp address, without leaking the legit one. Now _that_ would be something that doesn't exist yet - at least not in an easy to use form. By "easy to use", I mean POP3/IMAP support and a transparent mechanism for the user.