The fact that Mozilla relies on Google for most of its revenue is no secret. The company is set to get around $1 billion, €773 million for three years of Google being the default search engine in Firefox. But Google is not the only company Mozilla has a relationship with.
In fact, Google is not the default search engine in all the localized versions of Firefox. There's long been a Yandex version of Firefox and Yandex is the default in Russia and Russian speaking countries.
More recently, Mozilla partnered with Microsoft, once its arch nemesis, to offer a Bing-themed version of Firefox. Bing, of course, is a default search option in Firefox.
Now, Microsoft and Mozilla are partnering once again with a MSN-themed version of the browser, for the people that still use MSN for some reason. Probably the same reason why people still use Yahoo Mail.
This version of the browser comes with the standard modifications, Bing as the default search engine, both in the search box and the AwesomeBar, a link to msnNOW in the toolbar and MSN as the homepage.
Alternatively, if you've already got Firefox installed, you can grab the MSN add-on for the browser, for the same effect. If you're really into MSN, this "conversion" may be just the thing for you.
For Mozilla, the real draw is that this creates a different revenue stream. Granted, the sums involved are minimal compared to what Google is paying, but the company doesn't want its financial future to depend entirely on Google and for good reason, especially now that Mozilla is getting ready to launch its own mobile operating system, Firefox OS.
Another custom Firefox you may find interesting is the Twitter-themed browser, which was introduced about a year ago. Twitter is now a default search option in every Firefox, but this custom version adds more search options, making it possible to discover people or topics straight from the AwesomeBar or the search box.