But, it has been working fine until now, correct?
6008 has a huge number of possibilities including, heat, video drivers, faulty devices, failing HD's etc.
Have you checked CPU?
How to Check CPU Speed
http://www.liutilities.com/how-to/check-cpu-speed/
!) Uninstall Firefox
Reboot Windows
Reinstall Firefox
Perform the same steps you did up to the time the crash took place.
If everything works fine, you're done
- Please remove the antivirus and run the system for a period of time. If the issue does not occur, mainly focus on antivirus settings and compatibility.
If everything works fine, you're done.
- Temporary disable or unplug all USB devices, then check the result. If the issue does not occur, update drivers for them.
Right click on Command Prompt and select Run As Administrator
Type Shutdown /s
Press Enter
Once you reboot Windows, Check Event Viewer and verify whether the error appears when shutdown command was given.
Reset Power Options - About how to troubleshoot power plans in Windows 7
http://support.microsoft.com/kb/980869