EDIT: I had a gentle chat with the author, seems like he prefers no adfree version to be published. I don't have the will to keep a crusade on this app, so if you want it just do it yourself... sorry guys. It's not hard so the Fdroid guys will make it if they want to.
Ok, I'm doing it. I just forked NetGuard into "NetGuard Libre" (or "LibreGuard", suggest a name anybody?), I compiled the latest version removing Firebase.
Much still needs to be done, like maybe removing other unused/proprietary code that I missed, and fixing descriptions, names, etc.
NetGuard Libre v. 2.111alpha is running fine on my phone, I will post the online repository when it's ready. Contributions will be welcome.