This is another extension of the partnership between Mozilla and Microsoft. The two companies, often considered as rivals in the browser market, have partnered again to promote their products. Mozilla has released a customized version of Firefox with Bing as its default search engine and home page.
This special version of Firefox can be downloaded from the website FirefoxWithBing.com, which is hosted by Microsoft. For this version, Bing has been set default search engine in the search box & AwesomeBar and home page. Microsoft says about this special version, "When you add Bing to Mozilla Firefox, you’re bringing the ability to make informed decisions right to your browsing experience.".

Mozilla has partnered with several other search providers worldwide, and this is the main source of income of the company. Mozilla ships Firefox with Google as its default search engine in most of the countries including USA. According to Mozilla, there are nearly 20 such customized versions of Firefox distributed globally by partners including Bing, Twitter and Yandex.
Existing users of Firefox can always change their default search engine, and install search plug-ins to choose a new provider.