I haven't seen those before but scanning over the home pages of both it seems like we all offer very similar services. SimpleLogin seems legit and looks like they offer a ton of cool features. It seems like something that I would aspire this project to become
SimpleLogin [1] already does that and their aliases are more powerful. It's also open-source [2], can be self-hosted, and covers almost all platforms.
The Firefox relay seems to need SocketLabs [3], a paying and closed-source service. Haven't got my invite code yet but I would not bet on this Firefox service.