I found an odd issue with Google and Firefox today. I noticed that on some occasions, the first result on the SERP was GET'd by Firefox, despite the fact that I didn't visit the site. Cache was cleared, and it only happens when I use the firefox search bar.
(In listening to it myself again, I see that I messed up my lefts/rights when talking bout the windows... will be sure to pay more attention to that in the future :) )
Postscript: After some more digging and it appears that it is actually Google that is causing the prefetch. They do it by using the rel=prefetch attribute on links they want prefetched, and Firefox then goes and gets them.
That means that hits from Google could be inflated. For those with much higher traffic, the margin of error could be magnified.)