Provides a useful service, but implementation can be improved. Add-on allows you to generate a new alias from an email field, but this is a use case that is rare.

Much more common I would imagine is to choose an existing alias (from the maximum of 5 provided), and paste that alias into the email field (e.g. signing up for a newsletter). Add-on does not provide that, instead the user has to navigate to Firefox Relay web page, choose an alias, click on copy icon, change back to original tab, right-click the email field, and paste the alias.

6 separate actions, compared to one or two actions if you could right-click on the email field, move to the alias of interest, and click to select and paste.

Implementation is back to front (prioritising a rarely-used feature above an often-used feature) and needs fixing. UX seems to have been designed by developers, rather than humans (no slight against developers).