Where in Firefox can I make block these?
If you want to do this via Firefox, you could try using the add-on "Silent Block", I guess. I tried it out, and it seems to work although it's not well-documented.
However, I think that the original poster's idea would be a better one: change your DNS nameserver to opendns and customize your opendns settings to block unwelcome TLDs using https://store.opendns.com/get/premium-dns/. That will mean that all of your applications can take advantage of this protection, and it will be easy to set up for any other machines that you use.
(I actually run my own local DNS servers so I can easily block any TLD that I don't like, so I am not personally familiar with using customized opendns but the documentation indicates that this is easy to set up.)