Perhaps I solved the problem

How?
1. rename "D:\program files\mozilla firefox\uninstall\helper.exe" to helper.exe.old
2. copy helper.exe from the other copy of firefox to "D:\program files\mozilla firefox\uninstall"
3. double-click helper.exe, and firefox will be uninstalled

I don't know if there is some deep problem. I did see an "au_exe" file in the folder \users\admin\appdata\local\temp\~nsu.temp\, but it is not runing when checked in task manager and renaming it to something else did not encounter any problem.

How did I find the problem?

I cannot login to the administrator account (the problem was solved by regedit in safe mode), so I clicked around to see if anything went wrong, and I find two copies of firefox. I remember I installed only one, and I tried uninstall the new one, then I had the above problem.

Hope the information here can help other people.