Japanese is here.
Abstract Many programs include bugs and programming also includes debugging. Programmers develop their skill in debugging and establish their debugging methods through real debugging experience. Such skill and methods are applicable to other languages or bug manifestations. In recent years, web browsers become an infrastructure of GUIs as well as a common user interface of various network services. For example, Mozilla Firefox has the extension facility, which extends the function or interface of Firefox, and various extensions are developed by so many, perhaps amateur programmers. The users can install some extensions to improve their Firefox functions or to customize the view of certain web pages. Firefox extensions consist of both XUL/CSS for the GUI definitions and JavaScript for the actions. Many extension programmers say that none of useful development environments for extension programming exists and that the development is difficult. In this paper, we describe our experience of the extension development, its characteristics and problems, and the applicability of usual debugging methods or automated ones such as Delta Debugging.