No more need to precisely point at the tab close cross. No need to check if you are in a tab or a window before closing it. Streamlines the user experience, by example for users not familiar with tabs.
Tip: clicking in the top right corner (not just the cross) of your screen will usually close the current window, if it is maximized. With Fast Close Tabs, it will close the active tab.
The ALT+F4 shortcut will also close the active tab.
Since version 1.1, you can add a button to close the whole window. Use the "Customize" Firefox command (by right clicking on a toolbar) to add it where you want.
Since version 1.2, it works with Thunderbird 3.0.
Please let a comment if you like it.