In Chrome’s developer tools is was easy to see the width of a browser window. With developer tools open, grab to lower right corner and shrink the window. In the upper-right the size of the window shows up. That makes working with Media Queries so much easier.
I use Firefox so often and wanted the same feature. I found what I needed in firebug and this article helped me.
In the layout table as you inspect an element it’s size will show in the Layout tab.
I hope this helps someone else out there…