Firefox Sync actually allows this! https://github.com/mozilla-services/syncserver https://github.com/mozilla-services/syncserver This still uses FxA for authen

Firefox Sync actually allows this! https://github.com/mozilla-services/syncserver https://github.com/mozilla-services/syncserver This still uses FxA for authentication. You can self-host that up as well, but it's not nearly as straightforward, and I'm not aware of good documentation on how to do so.