Unless there is another user on here with the same setup and in China as you to verify what your seeing no one can know what China is doing with their internet access.

May you try: about:preferences --> General --> Network Settings --> No proxy If you're using FF in China Mainland with a VPN and can't connect to the Internet I think it's due to the proxy agent. It may needs administrator privilege to automatically modify the proxy settings. If not the case I'm sorry.