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This is not exactly an answer to your question, just my findings on the issue.

    <!DOCTYPE html>
    <html>
    <head>
    <title>TH Drag Test</title>
    <style>
    table,td,th {
        border: solid thin black;
    }
    </style>
    <script>
    	function OnDrag(e){
    		document.getElementById("lbl").textContent= e.offsetX + " : " + e.offsetY;
    	}
    </script>
    </head>
    <body>
        <span id="lbl"></span>
        <table>
            <thead>
                <tr>
                    <th draggable="true" ondrag="OnDrag(event);">Column A</th>
                    <th draggable="true" ondrag="OnDrag(event);">Column B</th>
                </tr>
            </thead>
            <tbody>
                <tr>
                    <td>One</td>
                    <td>Two</td>
                </tr>
                <tr>
                    <td>Three</td>
                    <td>Four</td>
                </tr>
            </tbody>
        </table>
    </body>
    </html>

And this

    <!DOCTYPE html>
    <html>
    <head>
    <title>TH Drag Test</title>
    <style>
    table,td,th {
        border: solid thin black;
    }
    </style>
    <script>
    	function Init(){
    		var n= document.getElementsByTagName("th");

    		for (var i=0; i<n.length; i++){
    			n[i].addEventListener('drag', function (e){
    				document.getElementById("lbl").textContent= e.offsetX + " : " + e.offsetY;
    			}, false);
    		}
    	}
    </script>
    </head>
    <body onload="Init();">
        <span id="lbl"></span>
        <table>
            <thead>
                <tr>
                    <th draggable="true">Column A</th>
                    <th draggable="true">Column B</th>
                </tr>
            </thead>
            <tbody>
                <tr>
                    <td>One</td>
                    <td>Two</td>
                </tr>
                <tr>
                    <td>Three</td>
                    <td>Four</td>
                </tr>
            </tbody>
        </table>
    </body>
    </html>

Both of these work in Chrome, none of them work in Firefox 17.0.1.