I have used the “Webdeveloper Toolbar” which is an addin on firefox, it’s really helpful to creating a great site
At my work (we do a lot of web-related stuff), the web developer extension is installed by default. It is one of the most useful extensions I have ever seen (rivaled only by IE Tab and the Extended Cookie Manager). Anybody who does web design in any capacity should get it. Just don’t forget to re-enable the cache if you turn it off… I spent several weeks wondering why YouTube and CD were loading so slowly when I accidentally left the cache turned off.
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
I also have that addin. That is really amazing too.
I use the WebDeveloper (EDIT: and Firebug) addin too, it helps immensely. I actually just discovered it, thought it would be useful, and decided to download it. It’s cool. 
Thanks man, Our website is pretty umm… in need of work.