A few days ago, Firefox 4 on my computer was displayed all fonts in bold style, that’s too bad. I though it because of monitor display setting. But it occurs only in Firefox 😐 So I asked Mr Google and found the reason. In Firefox 4, it has a new feature called Hardware Acceleration (GPU Rendering). When it is enabled, the font rendering is not correct (in some cases, Catalyst drivers for ATI graphic display is the reason), so simply turn it off to solve this problem by go to FF menu, choose Option -> Advanced -> General, untick Use hardware acceleration when available -> OK.