Google has famously been the default search engine in Firefox for many years. What few people know is that Google is not the default in every country. In places where Google does not dominate, Mozilla prefers to have the local favorite as the default, despite the monetary agreement.
For example, Yandex is the default search engine in the Russian version of the browser, Yahoo is used in some parts of Asia.
But that's about to change soon, apparently, Mozilla wants to switch from Yandex to Google with Firefox 14 in Russia.
There's no explanation for the move from Mozilla so we can only speculate. It could be that Google is paying well for the position, useful especially since it's far behind Yandex in Russian-speaking countries, however Mozilla hasn't made too many decision based on monetary needs before.