Try a set up like this: The back wall replaced with a low intensity area light.

The problem looks like the same one most GI routines have. They need lots of light!
The examples you linked to were all bright daylight.
The ones with the corner of a kitchen were just that, no ceiling or other walls!

In fact, the scenario you're attempting is challenging no matter which app. you're in.

You found the solution too, either increase the light or increase the sampling.

Putting in shadowless area lights, taking away walls and ceilings, reducing polycounts, excluding transparent objects from the GI calculation... These are the usual work-a-rounds and fixes you have to use in ArchViz.

Its probably a bit unfair to compare firefly unless you use all the same techniques!

A few renderers have better (and faster) sampling routines. Thats why architects, having spent thousands on a program, spend some more for Vray.
(The new C4D now does GI sampling in a similar way to Vray.)

regards
prixat