If you have ever tried the popular browser Opera, you have probably noticed it’s abundance of features. Probably one of the first ones you noticed was the download manager was in a tab. If you happened to like it, but don’t want to move away from firefox, there is a firefox extension which forces the Downloads window to display as a new tab. It saves space in your task bar, and lets you see more of your downloads in a neat little tab, instead of changing back and forth between windows.
Downloads in Tab [addons.mozilla.org]