Firebug is an excellent plugin for Firefox. But if you already have an error message, but don't know what to do with it, another tool may not help you. What is the error, and what is the code doing when it occurs?
'tempRows.cells[...].getElementsByTagName is null or not an object
That tells you exactly what is wrong, you are referencing something that does not exist. If you want to go old school, add alert statements and see what is going on there. I would guess that cells[x] is the wrong index.