If this is with the Google search engine in Firefox then there is the manual way when you search for something. Say you are searching for Windows NT but do not want Wikipedia .org results to show up. You can add multiple sites to exclude.

Windows NT -site:wikipedia.org

Thanks - I realise that, but just wanted it to be set automatically, so I do not labriously have to manually exclude wikipedia each time I search

Hello

Add or remove a search engine in Firefox. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox Add a custom search engine in Settings.

You can test.

https://www.google.com/search?q=%s -site:wikipedia.org