I am quite sure that backwards compatibility won't be an issue. As you may know Firefox OS is aimed at low-end phones, so I don't think they plan on ending supp

I am quite sure that backwards compatibility won't be an issue. As you may know Firefox OS is aimed at low-end phones, so I don't think they plan on ending support for first generation phones… ever?. The only issue is the manufacturer, which has to take care for the Firmware updates for his own handsets. ZTE doesn't look very eager to give its users constant updates.