This phone runs Nokia's Series 40, which is running 1.5 billion phones - supposedly the largest mobile operating system. Perhaps this is a bad example though as

This phone runs Nokia's Series 40, which is running 1.5 billion phones - supposedly the largest mobile operating system. Perhaps this is a bad example though as S40 only runs Java ME & HTML5 apps. Still, I can easily see these feature phones running Firefox within a year. Firefox is, after all, open source.