Phew! This one bug took me hours to track down the root cause. Which is this: If you set a non-standard attribute on a dom element, say _eventID, like so:
$('myDiv')._eventID = 12
Now, the _eventID attribute will be displayed inside the innerHTML of any of its ancestors in IE, but not in FF:
$('myDiv').parentNode.innerHTML
in IE you'd see something like:
...
in FF(keep in mind the value is still saved, just doesn't show up in the innerHTML):
...