After playing a bit with Firefox 4.0b6 I have noticed that something was different in the new Firefox, and that was caused by the font rendering used in the UI.
I know that Firefox is using Direct2D to render the UI on Windows 7, but in this area is not necessarily a step forward, and I hope that there is something to be done in this aspect (I have already given feedback using the build in feedback system)
I have made screen shots of Firefox 3.6 vs Firefox 4.0b6:
I have also done screen shots of Internet Explorer 8 vs Internet Explorer 9beta:
Notice that Internet Explorer 9 doesn't seem very different than Internet Explorer 8. I'm not sure that they did a better job at using Direct 2D than Firefox because they might not use Direct2D for menus all together.
This reminds me how broken was text rendering in WPF applications, but Microsoft seems to have fixed that in WPF4, see this blog entry about Visual Studio 2010
Also worth mentioning is that I have used the latest NVidia drivers for my graphics card:
Edit: This behavior was reported as bug #594325