If you've also got Firefox 3.6 or Chrome installed, compare the rendering side-by-side and you should see the difference.
There is also http://support.mozilla.com/en-US/questions/791489 and some other web pages reporting this problem.
There is also http://support.mozilla.com/en-US/questions/791489 and some other web pages reporting this problem.
Looks like for most people the problem is that FF4 is now using direct2d DirectWrite for font rendering where available, and DirectWrite has some issues.