List Domain Blocker od TheWebDev

Blocks requests to any webiste that are in user-defined list.
Visit about:addons, open the Domain Blocker's preferences, and mention the hostnames in the text box, you want to block. They should be separated by new line.

Note - The hostname-matching is very simple: hostnames must exactly match an entry in the list if they are to be blocked. So with the default settings, "example.org" would be blocked but "www.example.org" would be permitted.