Firefox OS has a huge advantage when running on low-memory phones. The most memory intensive thing typically run on a low-end phone is the web browser. (Or an app that wraps a browser, like Facebook). So the ability to run a web browser is the limiting factor. Every other OS has to have enough memory to run a GUI + a browser. Since Firefox OS uses the browser for its GUI, it theoretically can run smoother in 128MB or 256MB of RAM than any mainstream smartphone OS. IMO, Firefox should concentrate on the areas where it's intrinsic advantages lie.