Firefox might rule with programmers, but average users use IE. Nobody uses Opera except programmers (who wants another set of banner ads bugging you).
I have attempted to convert my office colleagues (non-programmers) to use Firefox, but they just keep on going back to IE. Reason: lots of current websites have features that break in Firefox.
Nobody's mentioned Safari ... most of my clients are graphic designers on Macs. This is yet another variant of Mozilla and has another set of things that don't look/act the same as Firefox ... in fact, it seems to work closer to IE than anything else!!
So, it's all well and good hoping that IE will go away, but if you want to keep most of your users, you have to code for it. And it's all well and good to conform to W3c, but if that means it doesn't work in IE, then I personally would reject the standard and provide something that does work in IE, w3c or not. That way, I stay in business.
And anyway, as someone said already, good solid coding will work on IE, Firefox, Opera and Safari.
Paul 🙂