What OS do they run? It's hard to imagine porting to another OS with insanely tiny marketshare and weird APIs. What is the app publishing ecosystem on these phones?
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.