im unable to use Fox search engine.

Firefox whether for Android, iOS/iPadOS or for desktop WIndows, macOS, Linux are web browsers and not a search engine site. A search engine is for example qwant.com, duckduckgo.com, google.com, etc as the Google search you are seeing in Firefox is as a search engine. To change or add search engines in Firefox web browser for Android see https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android