Is there a Yahoo toolbar still lurking somewhere? Check in View menu under toolbars.
You can also check to see if a plugin was put in under Tools>options>download and click on the plugins button at the bottom right of the window. (not likely, but can't hurt to look there)
Do you have yahoo selected as your search engine here: Excerpted from
http://www.mozilla.org/products/firefox/search******************
You can use the Search Box feature with many other search "engines", not just Google:
* If you click the icon at the left side of the Search Box, a list of engines drops down. Click on the one you want and that will become the current engine. Thereafter, whenever you press Enter to start a search, the search will use that engine.
* To add a new engine to your list, click the icon at the left end of the Search Box and select 'Add Engines...' This brings up the Mycroft page from which you can select additional engines.
******************
Edited for wording.... Sounded a bit smart-assed when I read it so I changed my wording... sorry about that....