I find IE8 slow to me, and IE7 even slower. This is after having tons of tabs opened as I tend to have. And there's quite a delay when opening a new tab too, which doesn't happen with Chrome. (comparing the two, since they both open a new process for each tab)
Memory usage? After having tons of tabs opened, Firefox is better for me. Otherwise is IE + IE + IE + IE +...+ IE (same with chrome), although yes, one tab won't crash them all. This is in "rush hours" and for performance, Firefox also wins for me in "user experience" as opposed to benchmark (chrome may too, but I miss ma'lovely addons.. T_T).
As for standards... yeah sure, was and has been so compatible that requires IE compatibility mode since websites had to be made with lots of tweaks to look properly on IE over the ages. (which is why has more websites requiring IE, for the "ages" part)
Security?... Blame the user for most of that stuff. =_="
#EDIT:
Forgot, Blackhawk, there have been 64bit Windows build of Firefox since a long time ago. Of course, they come with all the 64bit troubles and are completely experimental (which is why is called "Minefield" XD)
Mozilla x86-64 Builds
Have fun!