make firefox my search engine

Firefox for Android and iOS/iPadOS and desktop version for Windows, macOS, Linux are web browsers and not a search engine. A search engine is for example duckduckgo.vom, bing.com, google.com etc. To use another search engine in Firefox see https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android If you are actually asking for how to make Firefox your default web browser on Android then see https://support.mozilla.org/en-US/kb/make-firefox-default-browser-android