• by Laurie Sullivan  @lauriesullivan, July 24, 2015

Yahoo's agreement with Mozilla to become the default search engine on the Firefox browser for mobile and desktop might not become as profitable as first expected. The deal initially provided Yahoo with a spike in searches, but has since declined, according to data that analyzes about 1.95 billion unique visitors per month.

The analysis from AddThis, which aggregates data from more than 15 million sites, suggests that the proportion of Firefox users in the U.S. who conducted a search on Yahoo was 0.38% in October 2014, rising to 1.10% in December and then falling to 0.06% in January 2015, and 0.08% in July. The drop from October to January was 83% as a proportion of users searching on Yahoo.

As the numbers reveal, Yahoo searches saw a more modest decline of 10% in the U.S. during the October to January time frame. Since June, AddThis observed a slight uptick in U.S. Yahoo search activity on Firefox browsers, but it is still about 80% below pre-change levels. Searchers using Yahoo in general have seen a more modest 26% decline from October 2014 to July 2015, according to the data.

A Yahoo spokesperson said the company doesn't comment on this type of third-party data. 

In November 2014, Yahoo and Mozilla announced a five-year partnership. Yahoo would become the default search engine for the Firefox browser in the United States on mobile and desktop. The agreement also provided a framework for exploring future product integration and distribution opportunities to other markets, per the companies.

During Yahoo's second-quarter 2015 earnings call with analysts, Yahoo CEO Marissa Mayer called the deal with Mozilla "profitable" right away in Q1. "Click-driven revenue in Americas was up 24% year-over-year, resulting from paid-click growth of 24% through Mozilla distribution and other mobile partners," she said.