Complete unwanted power down

[LeonD]

My ibook randomly shut itself off whilst using Firefox in the YDL 6.1 distro, my fan was on overdrive which seemed odd as the machine itself wasn't overly hot (certainly nowhere near the temperature it reaches before the fan comes on in OS X). I checked on this forum and read an old post advising that someone had experienced problems with overheating and Firefox in the past so I switched to using Konqueror, the same thing happened today, just web browsing, fan comes on and then complete shut down, no warning, no reboot, nada?!!

It can't be the hardware as I experience no problems when running OS X, in fact I've had the laptop 3/4 years now and this is the first time I've experienced that!

Any ideas?