I would look in two different, but probably related directions. The first would be the proprietary graphics driver. These tend to be a buggy pain, regardless of the manufacturer. Though I have had better luck with Nvidia over time. You might try an earlier version, or a later version of the driver and see if that helps.
The other problem could be Fire Fox itself. I am becoming more and more dissolutioned with FF. I am afraid they are squandering their hard fought gains of the past year or two. The later version 3 betas seem to be more stable than the 2.0 series, this really shouldn't be.
Unless you are dependent on some particular extensions, I am almost to the point of recommending going to Opera. It appears to be a much better browser. And that hurts me to say.