https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android

Firefox is the web browser as Duck Duck Go is a search engine site.

Can you expand on your explanation of the difference

A web browser is a piece of computer software - AKA application, app - that retrieves web pages and linked content from a server over the internet (or some other network, but usually the internet) and then composes and renders the page and its linked content to display it on your device. Firefox, Chrome, Edge, and Opera are examples of web browsers.

DuckDuckGo is a search engine; search engines are systems that automatically scan content available on web servers, compile databases of information about that content, and then allow end users (you and me) to search the database looking for particular content. Google, Bing, Yahoo, and DuckDuckGo are examples of search engines. Many of them are accessible through websites - so you often will use a WEB BROWSER to access a SEARCH ENGINE, but they are not the same thing - and they may also be accessible through other means, like Firefox's integrated search feature, which allows one to query a search engine without first loading the web page interface (results are still displayed on the web page).