It's to be expected. The people who keep up to date with technology news and thus know FF4 and IE9 released are the ones that are more likely to know that the browser with the best add-on support is infinite times better than any browser MS can release, and so they update FF more than they update IE.
For example, I updated to FF4 on launch day, yet I didn't bother to update to IE9...in fact I don't think I've launched it since I formatted the PC and used it to dl Firefox, I prolly will update when it pops up in Windows Update though








