In all honesty I have not seen the same blurriness in IE9, although that maybe due to the fact that I am still using the RC.
I have fully patched Win7 and latest stable versions of everything (Drivers, IE9, FF4).
FF4 and IE9 both render identically in GDI (IE GDI = FF GDI) mode or DirectWrite (FF DW = IE DW) mode. I have tested them side by side on several pages.
The difference, Microsoft has a complex flowchart to decide when you use DirectWrite and other modern features, so it will often drop into IE7/IE8 compatibility mode. But you can force it into IE9 to use DirectWrite (or force IE8 rendering to render using GDI).
Find a page with Blurry text in FF4, load it in IE9, if it isn't identical hit "F12", then check the bottom Developer box for "Document Mode" Chances are it won't be in "IE9 Standards", go to the dropdown and select "IE9 Standards" and you will activate DirectWrite and text will be identically blurry to FF4.
I have found a way to check for
DirectWrite functioning independent of perceived font blur.
Find a page with reflowable text, not all of them work, but this one does so you spot it the first time:
DirectWrite « Marc Gregoire’s Blog
Now start shrinking the page down horizontally to squish the text, very slowly.
DirectWrite on: Shrinking is very smooth and even until a word gets moved.
DirectWrite off: Shinking jitters all over the place as words shake back and forth.
Again both IE9/FF4 behave the same way. Use F12 Document Mode to switch IE9 (no restart) or HW Accel off/on for FF4 (restart needed).
IE9 ie easier because you can switch without restart, but the are both the same.
Part of the point of DirectWrite is that it ignores pixel boundaries so movement, transformations are very smooth(but can have softer text). GDI text respects these boundaries so it has to jump to the grid (but can have sharper text).
FWIW: I downloaded the FF x64 Minefield 4.2a1. It behaves just like IE9/FF4 x32. DirectWrite is functioning and it creates some blur on some fonts.