the content area is defined by #container and inside of that #content
the widget area is controlled by the .main-aside class style.
i'm not 100% sure w/ the left-hand column, but i think there is some margin on the #container? or maybe it is #content and the rest is just the widths of those elements.
sometimes with firebug you can't point at the exact element, but you can get close that way and then pick the precise element in the left hand window that shows the HTML source. it helps that as you hover over the markup, the element in question is highlighted on the screen. clicking on a tag in the markup brings up that elements CSS in the right-hand window.