Hi. There is an option under Settings > Search where you can select one of the pre installed options, I use DuckDuckGo and you can delete those you do not want.

The option under Settings > Search.

The Yahoo search engine likely got added by an Extension install or update or by a external software. The desktop Firefox web browser has not come with a Yahoo search engine as one of the default options ever since the Firefox 57.0 to current Releases. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox https://support.mozilla.org/en-US/kb/add-custom-search-engine-firefox https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox