This issue alluded me at first as it works in IE but not in FF. See code below.

//pass in table, last cell number and style to apply to it. call this on hover and blur for cell highlight effects. alternatively you can determine last cell number as well and this function could be rewritten to work solely for the purpose of modifying specific cells rather than last cell
function ChangeTableCellStyle(tableid,cellnumber,mystyle) {
if (document.getElementById)
{
var selectedElement = document.getElementById(tableid);
selectedElement.className = style;
//change style on end cell by drilling into table. this will become deprecated by css3.
if (selectedElement.tagName.toLowerCase()=="table")
{

var tbody = selectedElement.lastChild;
if (tbody!=null)
{
var tr = tbody.lastChild;
if (tbody !=null)
{
var tr = tbody.lastChild; //BUGGED IN FF!

//nodetype should be 1 for element type. in FF it is 3. see reference link at bottom for list of types.
if (tr.nodeType!=1) {
tr.tbody.getElementsByTagName("td");
tr[cellnumber].className+= ' ' + mystyle;
} else {
tr.lastChild.className+=' ' + mystyle;
}

}
}

}
}

}

//example usage
ChangeTableCellStyle("table1",3,"cellend"); //will append the class cellend to the last cell in table1 if table1 only has 4 cells per row

In you’re interested in reviewing other approaches to styling your table cells, see my similar article here.