Mozilla has announced its plans to update the search options in Firefox 4, the next major version of the popular open source Web browser. Google will remain Firefox's default search engine, but Microsoft's Bing search will be available as one of the standard options. (...) Mozilla says that the decision is a response to the increasing relevance and maturity of Microsoft's search engine and growing demand for the service among Firefox users.
Mozilla has established a referral revenue sharing agreement with Microsoft so that it can get a cut of the search profit. He declined to say, however, what percentage Microsoft offers or how the deal with Microsoft compares financially to the one that Mozilla has with Google.
it's not that difficult to add Bing as a search option
I would agree, but many users wouldn't. :) I think that the revenue-sharing deal with Microsoft is the more important aspect here (I believe the plugin doesn't get revenue for Mozilla). Mozilla needs to reduce its dependence on Google for revenue and Mozilla executives have been talking up Bing's significance for a while.
If MS is happy to do deals with the open-source wordpress.com, then I wouldn't be totally surprised to see Bing supplanting Google as the primary search option for Firefox in a future version. :)