The original driver for the NVIDIA Geforce 8400 GS is from 2007. I have had a number of severe problems with Firefox 6.0. The Firefox error log suggests that the graphics driver is outdated and therefore a number of listed functions are inactivated, like web-gl. The driver should be replaced with the most recent one. Is an update imminent from Sony or can I just replace the old one with one from NVIDIA?
Vaio graphics drivers are rarely updated unless they cause major problems so don’t hold your breath on this.
You can, at your own risk, install the latest driver from NVIDIA but you will need a modified .inf file or the install will fail. Have a look at the following link for both the driver and modified .inf. Be sure to read the Quick Start Guide towards the bottom of the page.