Here\'s my problem: I\'m using font-face for the menu of my website, but Firefox on Mac displays it too bold On a PC, everything works well, the font is perfectly sized and
Every font supports different weights. Some can be really thing, others can't.
Mac and PC will render the same font differently. It's a core operating system thing, and basically, there's nothing you can do about it. :(
Most consistent results can be achieved using something like Google Webfonts or TypeKit.
One of the thinnest fonts standard on Mac is Helvetica Neue.
Also worth noting that font-weight supports values 100,200,...800,900.