To change the default search in Firefox for Android to Yahoo!
- Enter "about:config" (no quotes) in the address bar
- Enter "search" (no quotes) in the "Search Settings" box to filter the settings
- Locate the "browser.search.defaultenginename" setting and press "Modify"
- Enter "Yahoo!" (with the exclamation point, but no quotes) and press "OK"
- Your default search is now set to Yahoo!