A friend recently pointed me to a useful development toolbar for Firefox. It’s similar to the development toolbar here but does even more stuff.
You can find the toolbar at http://chrispederick.com/work/webdeveloper/.
I’ve recently been trying to make some of my scripts work with Firefox 1.5 (from IE) and I was looking for some debugging help to set me on the right track. I found this tool which seems to work very well: http://getahead.ltd.uk/ajax/venkman.
It’s an updated version of Venkman fixed to work with Firefox 1.5.
One of the things it helped me find is that whereas in IE if I ask a table cell (TD) for the nextSibling and get the next TD, in Firefox you always seem to get a text node first and then the next TD. Similarly for getting the next and previous rows.