Blue screen while web browsing

In the past few months my macbook pro with snow leopard OS X 10.6.8 has had issues while I've been using Firefox. While web browsing, my screen will suddenly go solid light blue (like at startup) and nothing will revive it except turning the computer off or shutting it, letting it go to sleep (which doesn't always work) and then reopening it and bringing it back out of sleep, at which point it goes back to where it was before the screen went blue. It seems to always happen when I'm clicking to go to a new page on Firefox or trying to exit a page or close the browser.

At first this rarely happened, but the frequency has been icnreasing to the point that it happens at least once a day now.

Anyone have any idea what this is? what's causing it etc?

Thanks!