Hey,
I use firebug:
https://addons.mozilla.org/en-US/firefox/addon/1843
It's an extremely comprehensive developer add-on that allows inspecting of html structure, the dom tree, script loadings and pretty much anything. You also get a nice console of all of your AJAX requests and errors. It does all of this on the fly, so if something is modified via AJAX you will know.
I don't know how I used to develop without it. It does slow certain sites down a lot so just enable it for the sites you want.
Hope that helps,
Ken