Firstly go to the Nvidia contol panel and go to manage 3D settings , change the power management to Prefer maximum performance , leaving it on adaptive is known to cause this problem. Open Firefox and see if it crashes.
If it still happens then remove all your Nvidia drivers , download the latest ones from Nvidia and do a clean install.
If it persists it sounds like the gpu render for firefox is making you crash. I don't use firefox , but check in the Firefox/internet options if you can disable it to software render instead, I know this option is available in IE9.
I should of said that it doesn't happen every single time. I'm usually on firefox all day, and it will happen once or twice, or sometimes not at all. I checked my control panel, and power management is already at max performance.
I checked and there is a new driver available for me. So I'm going to try that first. If it continues to happen then I'll uninstall my drivers.
I also went ahead and disabled the gpu render in firefox. So hopefully it gets fixed.