It would be nice to see more information about optimizing for IE and Firefox's JS engines, but to be fair, V8 accounts for a large percent of browser JS and all of Node.js as well.
Node.js is the answer, here. Most stuff you do on the client (outside webgl and game calculations) shouldn't be impacted heavily by these kinds of optimization snafus. But if you're trying to get your server to be as performant as possible, these can really kill you if you snafu naïvely.
Just a cursory glance at browser stats seems to show chrome as the most popular which would make sense. I highly doubt anyone using internet explorer is coming to SO.
Umm, why not? I use IE 11 in day-to-day usage and as my primary dev target. Chrome is god awful on my machine (it sucks on high DPI displays, has terrible touch support, worse/slower UI responsiveness and animations due to single UI thread, etc). I only use it for compat testing.
Also nice that IE doesn't still require prefixes for established things like transition/transform. If only Chrome and others would get around to implementing other useful CSS3 features that IE has (like grid) we might be able to actually use them some day.
I think the general idea is that asm.js is both well understood and not something a human should ever generate, so why would anyone but a transpiler writer care?
I'm not sure why you're talking about asm.js (I guess you're implying that asm.js is the only Gecko-specific optimization?), but I just want to note that I find it pretty hard to find good documentation about how to code with it.
The "not something a human should ever generate" doesn't seem like a right statement : before a transpiler can generate this kind of code, a human has to understand it.
Well V8 is the one that matters most. More users use Chrome then Firefox, so it's more important on the client-side. And server-side javascript is probably going to be running on V8. Node.js, Google's scripting platforms, all run V8.
It's like complaining if a C++ performance article only tested with GCC. Sure, there's other compilers out there, but that's the big one.