4chan's CSS specifies various parts of its site to have the font-weight set to 800. DirectWrite correctly specifies font-weight 800 to be:
DWRITE_FONT_WEIGHT_EXTRA_BOLD = 800
DWRITE_FONT_WEIGHT_ULTRA_BOLD = 800

Websites using this font-weight appear to be much bolder than FF3.6 or with hw accel off, because for once font weights are being rendered correctly (all other browsers except IE9+ do it wrong).

This userstyle changes the font-weight to 700 for post names and "normal" for the post area in order to get the more familiar look for FF4 users with hw accel enabled; the assumption is that Moot did not intend for those bold fonts to be "extra bold."

For a more permanent fix please e-mail Moot and tell him to fix his CSS.