Defult browser

Mozilla Firefox or Firefox for short is a web browser. To make Firefox the default web browser on Android see https://support.mozilla.org/en-US/kb/make-firefox-default-browser-android If by chance you were thinking of search engines then this KB article explains how to change or add search engines in Firefox for Android. https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android