how to make Firefox my default search engine

https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android Firefox for Android, iOS/iPadOS and the desktop version for Windows, macOS, Linux is a web browser and not a search engine. A search engine is for example duckduckgo.com. google.com, bing.com etc. If you want to make Firefox the default web browser on Android then see https://support.mozilla.org/en-US/kb/make-firefox-default-browser-android