http://blog.mozilla.com/blog/2011/08/16/new-tools-in-mozilla...
Also, it hasn't landed in Nightly yet, but there's some really slick dev tools that are being built into Firefox:
http://hacks.mozilla.org/2011/07/tilt-visualize-your-web-pag...
There's some nice CSS and Javascript tools coming too, but I can't seem to find any URL's on them at the moment.