When comparing Relay with the other two, I get the impression that SimpleLogin/AnonAddy actually interact with the community and understand that self-hosting is something people want to do and provide approachable documentation and support for that, whereas Firefox Relay seems built only with a single global prod deployment in mind and is more like "well theoretically you could but you're on your own and who would do that anyway?". Even if it's public and under an open license, the intended audience is Mozilla internal, e.g. [2]
Like, if a user signs up for Relay today and in 2 years Mozilla sunsets it, the way things looks today I think it wouldn't be viable even for most seasoned self-hosters to migrate to their own deployment.
I do appreciate the work you guys are doing and I think the engineers seem to have good intentions, so don't want to be overly critical. But mentioning it as open source alongside SimpleLogin and AnonAddy comes with some major caveats IMO, and I'd wish that some more priority is put on keeping docs complete and up to date and making the stack realistically approachable for outsiders.
[0]: https://news.ycombinator.com/item?id=30727935
[1]: https://news.ycombinator.com/item?id=30315816
[2]: https://github.com/mozilla/fx-private-relay#optional-enable-...