Ahh, i tracked this one down. Its an example of when not to cut/paste code from samples online without testing...sigh.It wasn't an event problem, but an incompatability between IE's and Firefox's DOM implementations.

The core issue was trying to update some <asp:label> controls with subtotals, and discovering that the most common solution discussed on the web: innerText, doesn't work consistently. That is, to retrieve the value in an asp:Label control, you typically use:

var test = document.getElementById('<%=txtBaseline2.ClientID%>').innerText;

which works in IE, but will fail in Firefox because the innertext property doesn't exist there. Tt has a textContent property instead.

Thus any part of your code that has to manipulate labels (ie: any values you don't want a user manipulating directly), you need to replace direct access to document.getElementById with functions such as:

        function getLabelText(LabelID) {
            // required to work around differences between IE and Firefox...
            if (document.all)
                // IE
                return document.getElementById(LabelID).innerText;
             else
                // Firefox
                return document.getElementById(LabelID).textContent;
        }

        function setLabelText(LabelID, value) {
            if (document.all)
                document.getElementById(LabelID).innerText = value;
            else
                document.getElementById(LabelID).textContent = value;
        }

You call them as follows:

getLabelText('<%=txtHeadSub3.ClientID%>')

or

setLabelText('<%=txtHeadSub3.ClientID%>', "Some Value");

Your code is then operation using both firefox and IE. Note that other solutions exist using pure DOM as well, which may be necessary if implementations of pseudo standards change yet again...