After picking up a new ad-sorting platform, Yahoo has struck a deal to integrate its search into Mozilla's browsers for half a decade.

Yahoo's search, which was sold to Microsoft and then licensed back, will replace Google's engine as the default in the Firefox browser.

"This partnership helps to expand our reach in search and gives us an opportunity to work even more closely with Mozilla to find ways to innovate in search, communications, and digital content," says Yahoo CEO Marissa Mayer.

Mozilla and Yahoo's recent handshake has a larger impact on Google and Microsoft than it does on Firefox users. Individuals who prefer Google's search engine can easily lift Firefox's hood and switch out Yahoo for a rival.

While it's an obvious win for Yahoo's search efforts, Microsoft will also enjoy the additional patronage of Bing. Yahoo is halfway through a 10-year agreement in which it purchased the rights to brand Microsoft's Bing search engine.

Yahoo's Search leverages the company's pre-existing technologies and integrates them with Bing, which is why "Yahoo" and "Bing" both appear in the listing of available search engines. And as Yahoo's financial struggles continue, the deal with Mozilla shows Mayer still believes Yahoo can drive its core services without having to fold into AOL, as some investors have suggested.

For Google, the deal is like watching Mozilla skip rope and jab at a punching bag in a gym. Mozilla hasn't said it directly, but the one-time big dog in the browser market is exploring ways it can become more competitive with Google.

Mozilla is still working on its Android rival, the Mozilla OS. The deal with Yahoo has the potential to chip away at the mountainous revenue Google earns from its search engine business, a stream of income upon which Mozilla has previously relied.

The integration of Yahoo's Search into Firefox will be rolled out at some point in December. Mayer says the company's search engine will be overhauled for Firefox integration and those changes will be rolled out into the full version of Search in early 2015.

As part of the deal, Yahoo will honor Mozilla's "Do Not Track" policy. Users will have the ability to opt out of the tracking services that fuel targeted advertising.

"Search inspires us because we think it's something that will change and improve dramatically, and because fundamentally, search is about human curiosity -- and that is something that will never be finished," Mayer says.