Major news for both Mozilla and Yahoo. Firefox’s default search engine is switching from Google to Yahoo in the United States.

Yahoo Inc said on Wednesday it struck a deal to be the default search engine on the Firefox web browser on desktop PCs and mobile devices in the United States, replacing market leader Google Inc.

The deal between Yahoo and Firefox maker Mozilla Corp will start in December and is set to last five years. Yahoo’s Chief Executive Marissa Mayer said  “I’m thrilled to announce that we’ve entered into a five-year partnership with Mozilla to make Yahoo the default search experience on Firefox across mobile and desktop. This is the most significant partnership for Yahoo in five years.”

The change will come to Firefox users in the US in December, and later Yahoo will bring that new “clean, modern and immersive search experience” to all Yahoo search users. In another part of the deal, Yahoo will support the Do Not Track technology for Firefox users, meaning that it will respect users’ preferences not to be tracked for advertising purposes.

With millions of users who perform about 100 billion searches a year, Firefox is a major source of the search traffic that’s Google’s bread and butter. Some of those searches produce search ads, and Mozilla has been funded primarily from a portion of that revenue that Google shares. In 2012, the most recent year for which figures are available, that search revenue brought to Mozilla’s $311 million.

It was not immediately clear how the agreement will affect Yahoo’s existing arrangement with Microsoft Corp, whose Bing search engine currently powers Yahoo’s web search capabilities.