Pretty much everyone in India - rich or poor - has a mobile phone these days. However, unlike the service, phones cost just as much as they do elsewhere so the poor usually end up using crappy feature phones with limited data support. A $25 smartphone with full web support would make an incredible difference IMO.
Though maybe it has some clever caching system that reduces this?
You can think of FirefoxOS apps the same way conceptually you'd think of them on Android/iOS, except they are always written in open web technologies (HTML/JS/CSS).
App developers do have the option to provide "hosted" apps to the store (which is basically a regular web page but without browser chrome), however even in this case writing HTML5 apps that work offline is totally doable using existing cross-browser standards.
In any way, the developer is in charge of how much bandwidth he uses.
In fact, Spice and Intex are the manufacturers that Mozilla has partnered to produce the $25 smart phones.
[1] http://www.micromaxinfo.com/mobiles/smartphones [2] http://www.maxxmobile.in/mobiles/smart-phones [3] http://www.si2imobility.com/spicemobiles/ [4] http://www.intexmobile.in/Product.aspx?PCatId=3
Like someone mentioned above, if you can afford the data plans, you would never go for this phone. The biggest hurdle I can see for this phone will be providing access to the internet.
There is certainly a good chance that data plan prices will lower with higher demand.