Firefox is a web browser as Google in this case is a search engine.

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

And could you explain the difference ?

Firefox (a web browser) will load web pages that you request. A search engine is a specific type of web page. If you want to switch to a different default search engine then see the link provided above.

Firefox is a web browser used to view websites like this one support.mozilla.org

A search engine is say google.com, bing.com, duckduckgo.com, yahoo.com, dogpile.com among many others. https://en.wikipedia.org/wiki/Web_search_engine

Mozilla the maker of the Firefox web browser for desktop Windows, Mac OSX, Linux and for Android and shell for iOS devices does not have a search engine of their own.