The first step is to add Yahoo as one of your search engines. On a Yahoo page, right-click the address bar and look for an item at the bottom of the menu to "Add Yahoo Search". Then it should be available on the Settings page.

Does that work on yours?

Right mouse click does not come up with drop down list!!!

When you right-click the address in the address bar, you don't get a menu? On Yahoo, or anywhere?

Another way to add it is to use the optional short search bar. When the site makes a search plugin available, a green plus appears on the icon at the left end of that bar. Click the icon and add the search engine from the drop-down. You can add/remove the search bar from the Settings page: Add the Search bar to your Firefox toolbar.