I can't say for sure. My only experience with a browser OS was an early Chromebook. There it was possible to develop an app in an online IDE, then download and install it. (I had to do this to build a rudimentary text editor that could save locally, although I believe there are usable editor apps available now.)

If there's no way to do something similar on Firefox OS, then I see that as a much worse hindrance to the ecosystem than the income demographic they are targeting.