Firefox OS did not come out of "Labs" -- Andreas and Chris worked in Client Engineering, led at the time by Damon Sicore. I took all of engineering including server side pieces formerly under Labs led by Mark Mayo in January 2013.

Firefox OS, neé B2G, was a skunkworks project for sure, which needed shaver and me as exec sponsors to avoid being killed in the cradle.

KaiOS's success could be read as us being too early (but we always thought we were late, starting in 2011); or as Mozilla not sticking it out and working on the low end to make deals such as the JioPhone one and others which I think were under way with Verizon. Whatever the case, KaiOS pulled off the vision and used code and engineers from FirefoxOS, which counts as a win for the concept.

Edit: so if the "window was closing", did it re-open? I think rather the window was always open, some variable amount, and awaited Reliance Jio's amazing growth via free-for-N-months 4G data. Android never fit in the low end, so the window was always there at some width of openness.