Kernel task is the kernel of the OS - it's the main "brain" of the OS so you can't quit it or you'll crash the computer. Everything else in the system depends on it, so if it does crash it's what's called a kernel panic and is why when it happens it's not graceful - the system simply stops and says "restart your computer".
Very high CPU usage of that task suggests that one of the drivers in the kernel is playing up. The first thing to do after trying a simple reboot is to try and reset the PRAM and SMC (check Apple's support pages for your specific model on how to do this, but it's pretty easy).
If this doesn't solve it then reinstalling OS X might also help, and/or remove any third party kernel extensions you have installed, if any. Try running the system with no peripherals connected like USB drives, connected printers etc - anything hooked up by USB.
Your system log might also give some clues about why the kernel is flipping out.
Edit: high CPU usage on firefox or other browsers is usually plugin related, almost exclusively Flash content. If you have youtube open then expect very high resource usage in Chrome and Firefox due to Flash. It's pretty heavy in Safari too.