Somewhat important to note that its only the default in the US, India and a few other countries - presumably not the EU because of the whole Internet Explorer setting Bing as the default debacle a couple of years ago
"This move makes Google Mozilla’s default search engine in most of the world, with the exception of China, where the default is Baidu, and Russia, Turkey, Belarus and Kazakhstan, where Yandex is the default."
That is, the deal with Yahoo only applied in the US, India, Hong Kong, et al.
