Interesting, but not sure how this is different than resizing the browser (save for the ability to see the current dimensions and jump to various breakpoints). Would love to see things like a resource calculator that could run a simulation of wifi, strong 3G, and weak 3G and let me know which resources are potential bottlenecks. Cool to see FF trying out dev tools like this.
"Interesting, but not sure how this is different than resizing the browser..."
I'm pretty sure it isn't any different, but it's great to have this built in to the Web Developer tools instead of having to use a bookmarklet/website/resize the whole browser window to do this.
