Google used to be the default search engine in Russia too (though I can't comment on whether they were paying for that). When it became clear that Yandex generally delivered better search results for queries in Russian, Mozilla switched to Yandex.
Are you 100% sure Yandex has not provided any funding to Mozilla? "Following our review in 2014, Google remained the default provider outside the US, Russia, Belarus, Kazakhstan, Turkey, China, and other regions where we had search agreements."
