(Andrew Harrer/Bloomberg)

Google has been the default search engine for U.S. users of Mozilla's Firefox browser for 10 years. But on Wednesday Mozilla announced that it would be swapping out Google for Yahoo Search in a five-year deal.

Firefox users search the Web more than 100 billion times a year, making it the country's third most popular Internet browser. That makes this a big deal for Yahoo -- especially because it relies heavily on search-related advertising revenue. So it's no surprise that the company sounded excited in its blog post announcing the deal, with Yahoo chief executive Marissa Mayer writing: "This is the most significant partnership for Yahoo in five years and we’re so proud that Mozilla has chosen us as their long-term partner in search."

But no where in Yahoo's announcement did the company mention its search partner, Microsoft. Despite Yahoo's roots in the online directory and search market, the company inked an agreement to sell its search technology to Microsoft in 2009. Under that 10-year deal, Yahoo Search is now powered by Mircosoft's Bing. (Microsoft gets 12 percent of Yahoo's search revenues under the deal.)

However, reports suggest that Mayer, who joined Yahoo after spending years at Google, has long been dissatisfied with that Microsoft search deal and is looking for a way out -- which may be an option next year when the deal will have reached its half-way point.

Yahoo's share of the U.S. search market has been on the decline since the deal was signed, while Microsoft's has grown. According to comScore, in September 2010 Yahoo accounted for 16.7 percent of "explicit" online searches -- those that occur when a user types in keywords as opposed to those automatically generated by sites contextually -- in the United States, while Microsoft had a 11.2 percent share. Four years later, comScore reported  in September that Yahoo searches accounted for just 10.3 percent of U.S. explicit core searches, while Microsoft had 19.4 percent of the market.

Both services still lag far behind Google, which accounted for more than two-thirds of online searches during that same period.

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