FireFox is reporting that there is an error on line 543. When I check the page source I see that this function definition is where the error is:
function displayTextBox()
{
var objElement = document.getElementById('YesTextBox');
YesTextBox.style.display = 'block';
YesTextBox.style.visibility = 'visible';
}
It seems to be complaining about the two lines in here that are setting properties of the YesTextBox directly. Since you are setting the objElement var to this checkbox you should be referencing that instead like this:
function displayTextBox()
{
var objElement = document.getElementById('YesTextBox');
objElement.style.display = 'block';
objElement.style.visibility = 'visible';
}
Is this code that you added or modified yourself? If not please explain how you went about setting up this validation so that we can look into this further.