If you want to add this as a search bar option in Firefox, you have to visit one of these pages first and then a green plus will appear in the search bar menu

If you want to add this as a search bar option in Firefox, you have to visit one of these pages first and then a green plus will appear in the search bar menu to (for example) add DuckDuckGo Lite. Then in settings you can also make it your new default. [When did Firefox remove the option to just type in a URL for this?]