In Firefox one can set a Default Search Engine:
Click the top right "settings" icon and then Options, see attached.
Select Search and then choose your Default Serach Engine
It sounds like you have this set to Google. There is no option to choose a different browser for the search result.
If Google is the default browser it should return a list of results. It always does for me.
Can you give an example of what you're searching for and a screenshot of the Yahoo result?