I click on the Firefox icon (taskbar). It brings up a Google Web Page.
That means you are using Google.com as your default home page. That is NOT the same thing as setting the search engine.

Go to the "3 lines button" (top right corner) > Options
Click on the Home tab (left side)
Set the Homepage and new windows to https://duckduckgo.com/

If you just type a search word in the address bar, it should take you to DuckDuckGo (assuming you set the search engine correctly)