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
-moz-osx-font-smoothing: grayscale;
https://bugzilla.mozilla.org/show_bug.cgi?id=857142#c83
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!