I have recently started using the Web Developer extension for firefox by Chris Pederick and I wonder how I ever managed without it.
It adds a toolbar to Firefox with a huge array of tools for analysing, viewing, editing and validating whatever webpage you are viewing. I had other tools and bookmarklets that accomplished the same tasks, but having them all (and more) in one place is a godsend.
One of the most useful tools is the CSS editing window which allows you to edit stylesheets and see the results onscreen immediately – very useful for editing dynamic pages.
You can also disable stylesheets and javascript, view info about images and links, outline various elements, use a ruler to measure sizes on screen, validate the document… you name it, it does it.
So, it’s great for development, but also for seeing how other sites achieve their design and layout.
I highly recomend it! http://chrispederick.com/work/webdeveloper/
