You can change it in Settings -> Search.

Your UserAgent Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:109.0) Gecko/20100101 Firefox/113.0 You appear to have a Ubuntu build of Firefox 113.0 and going by that version this is likely due to your Ubuntu or flavour of Ubuntu distro version is EOL with no more package updates including Firefox. You can either upstate your Linux distro or learn the basics of using the Firefox tarball from https://www.mozilla.org/firefox/all/desktop-release/ as it gets internal Firefox updates from Mozilla. Btw Firefox is a web browser as a search engine is google.com, duckduckgo.com, bing.com etc. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox