<!-- Due to different browser naming of certain key global variables, we need to do three different tests to determine their values -->
// Determine how much the visitor had scrolled
function myPopupRelocate() {
 var scrolledX, scrolledY;
 if( self.pageYOffset ) {
   scrolledX = self.pageXOffset;
   scrolledY = self.pageYOffset;
 } else if( document.documentElement && document.documentElement.scrollTop ) {
   scrolledX = document.documentElement.scrollLeft;
   scrolledY = document.documentElement.scrollTop;
 } else if( document.body ) {
   scrolledX = document.body.scrollLeft;
   scrolledY = document.body.scrollTop;
 }

 var centerX, centerY;
 if( self.innerHeight ) {
   centerX = self.innerWidth;
   centerY = self.innerHeight;
 } else if( document.documentElement && document.documentElement.clientHeight ) {
   centerX = document.documentElement.clientWidth;
   centerY = document.documentElement.clientHeight;
 } else if( document.body ) {
   centerX = document.body.clientWidth;
   centerY = document.body.clientHeight;
 }

 var leftOffset = scrolledX + (centerX - 350) / 2;
 var topOffset = scrolledY + (centerY - 265) / 2;

 document.getElementById("pop_signup").style.top = topOffset + "px";
 document.getElementById("pop_signup").style.left = leftOffset + "px";
}

function fireMyPopup() {
 myPopupRelocate();
 document.getElementById("pop_signup").style.display = "block";
 document.body.onscroll = myPopupRelocate;
 window.onscroll = myPopupRelocate;
}