I _want_ to like this, but there are some issues. In order for this service to be more useful than anonaddy, it needs to have the ability to create unlimited aliases with labels so that you can identify which are which.
Use Case: Using a different alias for each website so that you know which site leaked your email to other sites.

Being able to create an alias straight from the extension would be great as well. This would allow me to create an email alias with the firefox relay extension and the password with the lastpass extension. This pair would be a match made in heaven.

The ability to reply to emails that originated from Firefox Relay would also be a nice feature.

I'll be keeping an eye on this for future improvements. It definitely has potential.