I think the point is that Android has become quite resource-intensive an performs very poorly on low-end devices. Firefox-OS is intended to be extremely light and to perform well on even the most anemic hardware.

> Android has become quite resource-intensive

Some Android apps that are part of the typical Google-logo Android user experience have become resource intensive. Dalvik, even with the addition of a JIT compiler, was initially targeted at a very low resource requirement, and, apart from the JIT compiler, which is highly tuned to working in small battery powered devices, there have been no fundamental changes to the Dalvik VM until the recent release of ART as a replacement runtime.

But how can a web browser be "extremely light" nowadays? It has to do all the complex layout rendering and JavaScript execution of pages that have ~50MB memory usage.

Opera Mini made a lightweight mobile browser by offloading some of the rendering and javascript to a proxy server.

which gives you some really fast offline apps... not.

Except experience so far hasn't borne that out; FFOS has been pretty low performing on low performing hardware. The web was not made to be a lightweight platform, and any attempts to make it be one are going to be long struggles.