comScore today released its U.S. desktop search share report for December. Unlike all the previous reports last year, it shows big difference: Yahoo has nabbed 1.6 percentage points from Google.

As you can see in the chart below, Google slipped to 65.4 percent while Yahoo jumped the same amount to 11.8 percent. All other search sites remained largely flat:

comscore_search_december_2014

If you haven’t been following search market share numbers in the U.S., 1.6 percent may not seem like a significant change. For months now, however, comScore’s reports show next to no movement for each search site. We’re talking a difference of 0.1 points or 0.2 points one way or the other, if that.

More than 1 percent is thus a huge difference, though it’s really no surprise given the five-year deal Yahoo struck with Mozilla on November 19 to become its default search engine in the U.S. Given that Google was the default search engine of choice, it also follows that all of the search share Yahoo gained, Google lost.

The deal technically only went into effect on December 1, when Firefox 34 released with Yahoo as the default search engine in North America (Yandex also became the default for Russia, Belarus, and Kazakhstan). As such, we may see the impact on the U.S. search market spill over into January numbers.

Firefox users don’t update as quickly as those on Chrome (though they do update much faster than IE users). So we may see Yahoo’s search share increase further in January as more people update to the latest version of the browser.

On the other hand, many may try Yahoo but ultimately revert back to Google, since Firefox allows for changing search engines with just a few clicks. So it’s equally possible that we may see Yahoo’s search fall back down in January.

In other words, we’re going to be seeing the effects of this partnership for a few more months, even without a spillover effect. We don’t know all the details of what Mozilla and Yahoo plan to implement over the next five years — already, Yahoo sites have started telling Chrome and IE users to “upgrade” to Firefox. It’s likely there’s more where that came from, and while 1.6 percent isn’t massive, it might just be the beginning.