We’ve come all this way and nobody’s mentioned Watir? Seriously?
Watir’s a harness program that can listen to/send OLE commands to IE (FireWatir lets you do it with Firefox, but I haven’t tried to use it yet) and more than any of the other tools (Fiddler/FireBug/Web Developer), Watir’s the one that’s made may day-to-day job a lot nicer. Gone are tedious steps to reproduce a bug in the web app and the back-and-forth with QA to get a clean reproduction, replaced with a Watir script so that I can drive my app and see what goes wrong where. So very, very nice.
I know you have posted everything seemingly from Windows, so this may not be the proper place to post this, but I will anyways…
I use coda to dev websites (front and back ends) and I love firefox (mainly because of the inspect dom and highlighting of elements, and the realtime css changes you can make.
My Point: I get the feeling a good Mac App developer could create a similarly operating application for building the html/css of a site, building it real time, with the ability to save it as you are making changes. Basically building a page in firefox but having it save to your filesystem with a few UI modifications.