As usual, it's a security fix; there are no new features or add-ons. However, this one closes—among other things—a particularly nasty security hole that attackers can exploit to run arbitrary programs on your computer by simply embedding malicious HTML code into a webpage.
Windows users: Pay attention to the right side of the menu bar (right below the minimize/maximize/close buttons)—you'll see a round button with an upward-pointing white triangle. That's your “Upgrade” button. Click and follow the instructions. Or go to the [ Firefox homepage ] and download Firefox 1.0.7 for Windows.