[upofadown]

Which data? We can be somewhat sure that they don't have access to the content of Signal or Telegram secret chats as long as we have verified the identity of our contacts.

After that, what data do you care about? Neither Signal or Telegram is intended to provide complete anonymity. That is a much harder problem. For Mozilla that would involve Tor. I don't think that Mozilla really has "servers" in the sense you mean.