Ok, this was driving me up the wall.  You’d use @font-face to load a funky, cool font into a web page and then you’d look at it in Firefox and it would look like every web font had “font-weight: bold” assigned to it.  And this only seemed to happen in Firefox on Mac OS X.  After trying half a dozen other hacks to try and fix this – none of which worked I might add – there is FINALLY a fix for this!  And here it is folks:

body {
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;

That’s it.

Source: https://bugzilla.mozilla.org/show_bug.cgi?id=857142