While I consider myself a WebKit man, it's good to see Firefox competing, The competition benefits everyone.
Unfortunately, IE is still quite slow. I think we'll soon reach the point where it's not the browser feature inconsistencies limiting what cross-browser JS apps can do, but the performance difference.
I think its already here for the bleeding edge. I have seen some (small) apps where they give up on IE purely for performance reasons (their users are happy for that compromise) - buts its ONLY for performance reasons.
Unfortunately, IE is still quite slow. I think we'll soon reach the point where it's not the browser feature inconsistencies limiting what cross-browser JS apps can do, but the performance difference.