Go to Settings -> Search -> Search Shortcuts (at the bottom), and view the search engines that are checked. Remove any search engines you don't want to use by clicking on its bar and then the Remove button. Now make a search. Is it still using Yahoo?

If your searches are still being redirected to Yahoo, open Firefox in Troubleshoot Mode (Hamburger menu -> Help -> Troubleshoot Mode...). Then, try to search again. If the issue is fixed, it is likely one of your extensions that is causing this issue. Open Firefox in Standard Mode, disable extensions one at a time, making a search after every change. If disabling an extension fixes the issue, remove that extension.