Funny how difficult it is to give unambiguous directions to another user. Try as I might it's easy to overlook other's take on things.
krs, the easiest thing for me to do is decribe what I do because it's here in front of me and suggest that you (or others) do that.
So, I don't use Firefox much but I do have it because in my experience it's good to have more than one browser. Why is another topic.
I don't use Google for anything really but I do have a Google account because I have gmail.
I do occasionally use the Google Search page mostly for image search;
Nor do I use the DuckDuck Go Search Page;
I'm guessing this is the page you logged into for the DuckDuck Go app. I don't have this either and I think the reason it requires a later macOS than what you are running is because it's relatively new for macOS and there may not be any earlier versions.
All I have done is change the default search engine in Firefox Settings > Search > Default Search Engine to DDG, as shown in my previous post. In fact I've changed my default search engine to DDG in all my browsers, including safari;
This means the top of my Firefox Home page (or any page) looks like this;
You can see that the search bar says, "Search with DuckDuck Go or enter search address" or as I'm on the home page it also shows DDG in the Firefox Search bar.
When I enter a search in the Address Bar I get this;
You can see, on the bottom of the results list I could, "This time search with" a different search engine if I wanted different results. This does not change the default, it's only for this search.
As you can see, no "Trending" results but bear in mind DDG will return different results to Google, as will Bing, Amazon and Wikipedia. They are geared to provide the "hits" they think you will want to see whereas DDG simply tries to avoid dodgy sites that may track you.