liliputing.com/2013/06/android-gets-cheaper-meet-the-100-quad-core-phone-59-dual-core-tablet.html
liliputing.com/2013/06/android-gets-cheaper-meet-the-100-quad-core-phone-59-dual-core-tablet.html
My guess is that Mozilla see the low end as the most vulnerable angle. Apple aren't going low end. Android aren't excluding it, but <$100 phones are an afterthought.
There is always demand at the lower end. Handset makers are happy to fill it. Android might be the best current option, but Google aren't focused on this segment specifically and I think most app developers are developing for high or midrange phones first. A lot of stuff doesn't work well on a cheap android. By focusing on less power and smaller screens, they will encourage developers to make simpler apps that run well on this hardware.
It's still early to tell if they have an advantage over android in this market, but it makes sense in theory.