You don't have to use a cloud server, just host it yourself.

Oh, you mean like Firefox Sync?

Which was self-hostable.. Promised to be that once again, but still hasn't been opened up again.

Well, it is self-hostable, but it's a giant mess.

I've self-hosted Accounts+Sync with a lots of elbow grease. Even had my own alternative re-implementation - because running all this Mozilla code was more complicated than hacking up something of my own.

I gave up about a half an year ago.