1. As far as the effect of the window manager: this isn't emacs+X11 in 1995: the drag on CPU/ram/battery of gecko/blink will dwarf the "weight" of the windowing system.
Firefox OS can actually run on a 1 ARM CPU with as low as 128mb. It may not be the best experience but Android and WP can't even boot given those constraints. Firefox OS can be really nimble, the main issue when people complain about performance is usually a poorly implemented app and not the OS.
2. FirefoxOS maybe be leaner in principle, but the competition has vastly better hardware, both in the Windows Phone field and in the Android one, via what I can guess is a combination of subsidies (is mobile MS still burning piles of cash or has the division posted some profits?) and sheer mass scale production. So, for little more money you get a get a better device that actually runs all the apps that your friends have, and a web browser.
That is true for some places but not everywhere. There are places where all the devices are imported and heavily taxed. In these cases Firefox OS tends to be a lot cheaper than a comparable Android. Some people can't afford an Android device, they may be stuck with dumb phones, in these cases, Firefox OS is an appealing solution. Thats the original case, migrating people to their first smartphone and give them web access.
3. The appeal of FirefoxOS even in those low end markets was negligible (this has been discussed to death). Besides they released phones in places such as Italy and Spain, two markets were people will en masse shell out the cash for an Iphone. I found that simply baffling.
I will not comment on the negligible part of your comment because I know people from those locations, real people not data points in a graph, who were delighted to have their first device. Also remember that Firefox OS is a partnership with hardware makers and carriers. The carriers decide where it will launch and both TIM and Telefonica wanted to launch on their home country. It was their decision, we may not agree, but its their call to launch on their home base.