I used Firefox Relay for several months on my Gmail account but then I found that although I could send emails to my friends at least three of them (possibly more) were unable to reply as the message they kept receiving was that the server was not compatable.

So ---- after much serching, I finally found (by mistake) the place where I could remove Firefox Relay. It was not easy to find this!!!

Now my friends can send emails to me again.

To answer the response from the developer: They were trying to send me emails to my regular Gmail address as they always had. They were not doing anything different from what they always had in sending me emails.