Yahoo! is not the default search engine for Firefox browser in the United States but they have specially coded their page to help users change their preference. When you first visit Yahoo.com a small box appears to help you switch from Google search to Yahoo! (as pictured above).
Pretty smart! I’m surprised the other search engines bundled with Firefox such as Amazon and eBay have not done the same thing for their power users even if they cannot be a replacement for all things search.