In the real world, Firefox OS has zero marketshare at the low-end. Mozilla announced just a day or two ago that they're abandoning the low-end strategy.
Featurephones are largely gone. Microsoft bought Nokia's featurephone business, shut down further development and has been converting it to low-end Lumias as fast as they can, but Android is nibbling away most of that market.
Mozilla only announced that they're abandoning efforts to build the lowest-end stuff. IIRC, most FirefoxOS handsets are still pretty low-end, and have been doing reasonably well.
And feature phones are still dominant in places like Africa and South America, particularly due to their lower cost and power consumption (smartphones are still unable to reach the battery lives of even dumbphones from a decade ago, which is an important consideration in environments with limited/inconsistent electricity). Even in "developed" countries, feature phones are popular with the elderly and disabled, since they usually feature physical buttons that are easier to work with (better tactile feedback, easier to find with poor vision, etc.) and have simpler interfaces.
Featurephones are largely gone. Microsoft bought Nokia's featurephone business, shut down further development and has been converting it to low-end Lumias as fast as they can, but Android is nibbling away most of that market.