I just discovered a nice add-on for the Firefox add-on Firebug.
You need Firebug for that: http://getfirebug.com/
It just behavous like Firebug and extends it with quite useful informations about the selected DOMs.
Combine it with another tool for jQuery https://addons.mozilla.org/de/firefox/addon/5369
(Yslow, see how quick your site is) and you should have some nice tools.