Malware attack warning in firefox by Google

Firefox uses Google's malware filter to block sites having potential malware. A webmasterworld member has reported seeing this. He seem to notice it only now though this has been there for a long time,

he says,

"G have long been warning in their serp that a site may harm your computer, but this is new to me...
My platform is Mac, FF, with Addons; NoScript, MacAfee SiteAdvisor, and AdBlock Plus.
I was NOT using any Google site or feature, but tried to visit two sites I regularly go to +from a bookmarks page on my desktop+.
The first was a Formula One news site, the other a cycling enthusiast site, nothing dodgy at either usually.
However, a semi-opaque interstitial warning page covered my browser view warning me that the site I'm trying to visit may harm my computer, and according to my setting I am being warned, proceed or not?
Instead of visiting, I clicked on the "More info" link and was taken to a Google url (sorry I didn't save the url) with data on the site's malware problem, and last 90 days scanning results. It seems they had some harmful script inserted in their ads by a third party.
I removed those sites from my bookmarks page.
What concerns me, is that later I realized that I hadn't used any Google feature to visit those sites, so how did G know I was going to them?
I didn't go through G search, or a G bookmark. I went through a simple homemade bookmarks page on my desktop.
I have a Google account that I remain logged into most of the time. I use it for sitemaps, and the Google removal tool, G alerts, and of course G search.
However when I looked in my G account settings for anything remotely like, "Warn me if I ever try to visit a dodgy site", and there's nothing there remotely like that. I use the default "moderate" setting to stop filth showing in the serp.
So, how come G popped the interstitial, when I wasn't using any G site feature I'm aware of? Too spooky.
All I can guess is that McAfee are collaborating with G on the Site Advisor feature, and extending it's reach beyond the G serp? Or perhaps it is the NoScript addon people who G are collaborating with?
Normally when the McAfee addon warns you about a dodgy site it will point you to an info page on the McAfee domain, not the Google domain.
Anyone else seen this recently? Was it a test run, a slip-up, or old news?
Interestingly, I visited the same F1 site today using the same method, and got no warning interstitial, nor did the G serp listing of this site have any warning note. "