Google just took me to http://www.nihilogic.dk, which is eventually a blog of the author of Pixastic. There are some comments in there mentioning my problem. The solution is that: you need to put the script and you "hello world" page for the script in a server. Therefore, Firefox won't complain about the security thing. I copied the codes to my localhost and it just works!
I would like to thank Jacob Seidelin for creating this great tool.