[arch-general] Arch shutdown because of firefox

hi all, I am a newbie for ArchLinux. My computer often shuts down suddenly when I browse web site. It brings me to TTY and said "The system is going down for system halt NOW!". I found that is because of the crash of flash plugin of firefox. And my question is why the crash of firefox can cause my computer shuts down? PS. my system environment: Arch + awesome, no DE. Thanks! hc