Firefox 3.5: "Authentication required"

Everyday I'm on http://dallas.craigslist.org/cps/

Suddenly tonight, I'm getting this...
http://files.getdropbox.com/u/560626/Pics/screenshot.jpg

I have to hit cancel twice every time I load this page. I've never even been to this www dot atrstore dot com before it showed up on my page.

I've cleared my cache (the "clear private data" menu option has disappeared now in 3.5 so I had to manually delete the files in the cache folder) and restarted several times. I've done a flushdns, for what it's worth

It is not the Last Pass extension, for that has been disabled (and that's not the last pass prompt anyways, but a Firefox one)

The website it suggests is something that can be found in the craigslist ads, a computer service website I just checked out. So how is it tied into my settings?

What can be done? I hope I can hear some real good advice and I'll post the working solution. thanks