It seems that Firefox is rendering fonts a bit too thick. font-weight is set to the right values, the problem is partially solved when decreasing the weight 100 (from 300 to 200) but is then displayed too thin on the other browsers.
This image shows Safari 6, Chrome 25 and Firefox 19
You also can see clearly that the Font Awesome Icons I used are too thick and don't look good at all. Also the font-weight is set correctly and is displayed correctly in the Inspector of Firefox.