One of the new features in Firefox 3.5 is the ability to "tear" a browser tab off into a new window. Say you have 5 browser tabs open, but you'd rather have one of those tabs open in an entirely new Firefox window. Just click on the tab and drag it away from the tab bar to launch a new window. You can also drag tabs from one browser window to another the same way.
But there are a few downsides. First, you can no longer drag a tab onto the desktop to create a shortcut. Second, it's easier than I'd like to accidentally tear a tab into a new window when you just wanted to click on a tab.
