The biggest advantages of Firefox were the tabbed browsing features and the built in search bar. When MS started losing market share to Firefox, they updated, but MS never innovates. Once they had vanquished Netscape they didn't do any significant upgrades to Explorer.
Windows upgraded Explorer to be similar to Firefox, so there was less advantage to switching. I didn't install the new IE, because Windows won't let you install IE without installing Windows Genuine Advantage.