The nifty Faviconize Tab extension minimizes the width of the label on a Firefox tab to just the favicon, to save space on your desktop. You can decide which URLs should alwas be "faviconized," and which ones should have their titles spelled out.
What's a favicon? Look to the top of your Firefox browser window right now. See the address bar? See the little "IW" immediately to the left of it? That's a favicon.