Maybe it was that Google has a competing browser, making them strange bedfellows. Or after ten years of partnership, it was time for change. Or maybe Yahoo just offered to pay more.

Yahoo is replacing Google as the default search engine for Mozilla's Firefox browser, the companies announced late Wednesday. With 10% of the market, it is the Internet's third most popular search engine, behind Google's Chrome and Microsoft's Bing (which powers Yahoo searches).

The change is significant for Firefox users, who perform some 100 million searches in the browser every year, according to Mozilla.

It is significant for Mozilla, a non-profit that is heavily reliant on revenue from search companies. Google's three-year contract with Firefox, which will come to an end in December, had reportedly been worth $1 billion. Yahoo and Mozilla did not disclose terms of their five-year search deal.