It's only Firefox on OS X for me. Every other browser, mac or windows, renders more or less the same but FF on OS X really lays it on thick. Like you, I've had no luck with assigning font weights
It will be
It's already in the nightly build.
In the mean time, this is what I figured out:
Making a slightly "lighter" version of the icons is the best way to compensate for this. They're passable in Firefox, and ever-so-slightly lighter in all other browsers. In Illustrator, I create a 16px icon. I resize it to 1024px, and apply an offset path of -6px (these are the numbers that worked best for me). I then export that as a svg, and import it into IcoMoon to build the icon-font. I don't use -webkit-font-smoothing:antialiased (or the pending -moz-osx-font-smoothing: grayscale) unless it's a light icon over a dark background. This is the best way I've found to get icon-fonts to display (mostly) uniformly across browsers & platforms.
-moz-osx-font-smoothing: grayscale; did the trick!
Thank you very much!