since recently Europe People are able to choose a different search engine app than Google in the Google Pixel Launcher. However, Firefox is not listed there. Please make the app to appear there.
Eventhough one can set Firefox as the default search engine in Pixel, I would say that a browser is useless in the pixel launcher, cause there is a blurring of the line dividing search and browser.
Pixel phones are set in a way that Google search is the only necessary app to browse the internet... So I agree that you should try to be included in the search app. And I do realize that you are not a search engine, but you have to adapt to the market, and truth is: for phones ,search and browser are now the same thing!
Since duckduckgo is the only privacy respecting search/browser app in android you will see a many people choosing duckduckgo over Firefox just for convenience...