I want to Merge all them.
so, I go to "Window > Merge All Windows".
but I use this function just from shortcut.
This topic is about how to set the shortcut.
1. System Preferences > Keyboard

2. add Shortcut Key, Click the "+" Button.

3. select an Application.
Safari in this case.

4. write a Menu Title and set a shortcut key which you like.
"Merge All Window" in Safari's Menu in this case.

5. so, just type a shortcut key, then my messy desktop will be ...

tabs in a window.