A: Firefox Relay is not currently designed to filter out spam, but does offer some basic spam protection based on platform it operates on. Built upon Amazon's Simple Email Service (SES), Firefox Relay, via Amazon SES, uses block lists from known spammers and also scans attachments for viruses, thus providing some basic protection from certain spam and malware. One of the benefits of using Firefox Relay is if a specific alias starts to receive spam, you can simply stop that alias from forwarding emails to your real email address.
Firefox relay also suggests to get in touch with them if you see a broader issue with spam (for example all your aliases are getting spam messages) since they may be able to make specific adjustments via Amazon SES.
Finally, it's not recommended to report relayed messages, to your real email address, as spam, since that would indicate your Firefox relay alias as spam, and not the original sender.