IE can be tweaked some to remove some vulnerabilities.
Interestingly enough, the number of Mozilla exploits have gone up lately. More users means more targets. It's not completely linear though, because Mozilla users tend to be more tech savvy than mom 'n pops with IE.
Oh and the "problem" on websites displaying incorrectly is NOT Mozilla most of the time. Rather it's web developers writing non DOM stuff, i.e IE specific code. Blame the coders there, not the browser.