"We believe that the reason behind [this change] was Mozilla's deal with Google," said Vladimir Isaev, manager of international media relations, in a Sunday email reply to questions.

In December, Mozilla and Google announced a that sets Google as the default search tool for Firefox. Although each company declined to detail the financial arrangements, reports claimed that Google had .

That amount, if accurate, would be nearly triple what Google paid Mozilla in 2010, the last year for which the browser maker has released revenue data.

According to Yandex, Mozilla notified it of the search change on June 1.

"We have [a] global partnership agreement with Mozilla and the default search in Russian version of Firefox is a part of it," said Isaev. "The agreement will expire on December 31, 2012. According to the agreement, Mozilla can change the default search [at] any time."