Firefox and IE, by default, use an odd hybrid approach where large fonts get DirectWrite but smaller fonts fall back to 'GDI' style rendering.

If you're using Firefox try these preferences out for a full DirectWrite experience:

    gfx.font_rendering.directwrite.enabled: true;
    gfx.font_rendering.directwrite.use_gdi_table_loading: false;
    gfx.font_rendering.cleartype_params.force_gdi_classic_max_size: 6;
    gfx.font_rendering.cleartype_params.force_gdi_classic_for_families: ''; (delete the string)
    gfx.font_rendering.cleartype_params.rendering_mode: 5;