Most versions of Android run very well in 256MB of RAM, even 128MB. The existing version of KitKit is not yet optimized for such little RAM, that will change. If it were Google's goal, they could have a 128MB version of KitKat ready next month. If Firefox OS sees much uptake, expect those plans to be accelerated.
If Mozilla is hell bent on creating a mobile OS, a better plan would be to recreate the closed portions of the Android ecosystem under the Mozilla banner. An open source version of Google Play services and store would be heartily welcomed by most of the Android community, while achieving most of the goals Mozilla has set forth.
In a direct battle against Microsoft, Apple, and Google, an app-less Firefox OS stands very little chance of impacting the battlefield. The BOM difference between an Android device and a Firefox OS device is just too small.