Then I came a cross this Scratchpad built into Firefox itself. I was not aware of this tool for all the time I am using FireBug for debugging. And I found this Scratchpad has a "Pretty Print" button. ^_^ How nice, a code beautifier is right there!
The output is good and I don't need to remember anything, except the shortcut "Shift+F4", I think I will use it a lot!