I have noticed that rendering of fonts differ considerably if the size is, for example. 11px. Running Windows 7
The following HTML and CSS is used
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Firefox 7+ under Windows 7 uses GDI Classic mode (with hinting) to render so called Core Web fonts like Arial (since they are more sharp and readable with hinting enabled), and DirectWrite (without hinting) for other fonts. Specific fonts to use GDI mode with are listed in following about:config pref:
gfx.font_rendering.cleartype_params.force_gdi_classic_for_families
AFAIK, rendering mode in Firefox depends also on font size. For enough-big font-sizes and too small ones (perhaps greater than 15px and maybe less than 9px), it uses DirectWrite for Core Web fonts too.
IE9 under Windows 7 uses DirectWrite always.
Chrome seems to use GDI classic mode always.