Do not go to them

I don't. I just want to have more control over the available content.

Give these a try : https://support.mozilla.org/en-US/kb/block-and-unblock-websites-parental-controls-firef https://addons.mozilla.org/en-US/firefox/addon/blocksite/

Please let us know if this solved your issue or if need further assistance.

Where do you see the websites that you do not want to see? For example:

On the new tab page: The Recommendations from Pocket users may be slightly customized based on your interests -- that's a work in progress -- but you still may see sources you don't like. As far as I know, you can't hide individual stories. To hide that entire section, see: Customize your New Tab page to hide or display content.

In search results: You can hide sites from search results using an add-on, such as my user script that I describe here: https://www.jeffersonscher.com/gm/google-hit-hider/