I have found an add-on called BlockSite (hxxps://addons.mozilla.org/de/firefox/addon/blocksite/?src=ss). It works but I'm not sure about the developer's reputation. His website (wips.com) has a poor reputation according to WOT.
Thanks for your suggestions. The add-on mentioned by art above is pretty close to what I'm looking for. Will give it a try. Thanks for all your suggestions.