Arvind just posted Javascript application testing - Firebug which talks about using Firebug to do some client-side manipulation on a browser.
What I like about that post is that he is using TeamMentor as the background of his example.
That is great, since one of the reasons why I fell is so important to have TeamMentor's source code publicly available is to allow its code to be used in application security threads (like for example the one I tried to start with Is there a spreadsheet/template for Mapping WebServices Authorization Rules? )