If you are looking for a country-specific Yahoo search plugin (e.g., UK, Germany, Japan), you can install them directly from the Yahoo results page, or as a shortcut, I created install buttons here: https://www.userchrome.org/yahooxml.html
Want to get rid of Yahoo search?
If you want to remove this search plugin from your Firefox, go to the Preferences/Options page, Search panel, One-Click Search Engines box, click "Yahoo.com Search" to highlight it, then click the Remove button below the box. Full details on this panel can be found here:
While you're there, consider whether you have another Yahoo search plugin, or McAfee (which uses Yahoo), or anything that you do not plan to use. Remove remove remove.
If you still have issues with your searches going to an unexpected site, post a question on Mozilla Support. They can help you find and remove search hijackers. The two usual problem areas are: (1) One of your extensions, or (2) A program folder infection. Good luck.