it is not cleartype, cleartype has been default since vista. it is hardware acceleration... both have been enabled on internet explorer and firefox for a while
It's not hardware acceleration, it's
hinting. (That link is very much worth a read, and the rest of this post assumes that you've read it or that you already understand what hinting is.)
Windows GDI font rendering is heavily hinted (it's why text on Windows looks so beautiful on low-DPI screens). However, the new hardware-accelerated APIs in Windows are, by default, unhinted (or very lightly hinted--I can't remember). This produces a more "faithful" text and text that scales more smoothly (e.g., if you have a text animation that grows the text from 10pt to 100pt, that animation will be smooth with unhinted rendering and will be jerky with hinted rendering).
When IE switched to hardware-accelerated text rendering, it got unhinted text. That's why some text looks a bit smaller than before, and why text don't look quite as sharp and clear as before.
When Firefox switched to hardware-accelerated text rendering, there was quite a bit of controversy, and a lot of people (including myself) argued that the benefits of hinting outweighed the downsides (most of the text that we see is static text for
reading--we should thus optimize for on-screen readability, not for print fidelity or for text animations because, well, we know how people just loved the blink tag, right?). The compromise that was reached was that there was a list of fonts that would get the classic GDI-style full-on hinting treatment. This included the UI fonts like Segoe UI and all the common core web fonts like Arial and Verdana. This list of exemptions is configurable and can be found in about:config under "gfx.font_rendering.cleartype_params.force_gdi_classic_for_families". However,
these exempted fonts still use hardware-accelerated rendering. It's just that Firefox is passing a parameter that says that it wants classic GDI-style hinting.