Now everyone uses Windows, hence Linux and OS X users aren't exactly in a position to switch to IE, regardless of its technical merits. This is no longer a Windows-only world, even if it's still the majority.
Besides, IE lacks the useful extensions I rely on in Firefox. Don't tell me said extensions are pointless or useless - I find use in them, so clearly they have worth. Going to IE would mean giving up said extensions or having to do things in a less smooth or capable fashion. Firefox is still the best browser for the power user who wants as much functionality and flexibility as possible. IE is for the conservative user who wants something fast and integrated well with the OS and doesn't have any particular needs or wants outside of what the browser itself supports.
Sure, Firefox has its deficient (the inability of Mozilla to use multiple cores for separate tabs is still worrying), but you take the good with the bad. There is no one good browser.