You know there is a world outside of the US right? And on a global scale, Firefox is only marginally ahead of IE6.
What is very apparent from those graphs (in so much as such things can be accurate) is that adoption of newer versions of Firefox is much quicker than adoption of newer versions of IE. Which probably fits in with the general consensus that Firefox is mostly
used by early adopter types and doesn't have much take up in enterprises, where a version change isn't something that happens lightly.
Also because Firefox completely drops support for older versions.