Your User Agent string, from the system info aside is identifying you as apparently using some sort of Firefox 3.6 That also has a rather suspicious element QQDownload/1.7 so possibly you have downloaded Firefox from some sort of unofficial site, and or have some odd software installed.

• to check your User Agent String yourself use
  Firefox Button -> Help -> Troubleshooting information
  • we see: Mozilla/5.0 (Windows; U; Windows NT 6.1; zh-CN; rv:1.9.2.8) Gecko/20100722 Firefox/3.6.8 QQDownload/1.7

I suggest you

• use a recognised Firefox site such as http://www.mozilla.org/en-US/products/download.html
• consider a clean reinstall for instructions see:
  http://kb.mozillazine.org/Standard_diagnostic_%28Firefox%29#Clean_reinstall
• try to identify what software you have that is causing this change, and if it is not essential consider removing it
  • see troubleshooting extensions and themes

Are you using a Chinese ISP or downloading a Chinese version ?
(I note the UAS includes zh_CN )

I have resolved the problem. It's the User Agent Switcher addon that cause that problem. I have ever installed the addon before and changed the configuration, but I uninstalled it without reset the configuration. So the user agent seems odd. When I reinstall the addon and reset it to the default user agent, it becomes ok. Thanks a lot for helping me!

Glad you sorted it out,
John