It's actually a script, not a popup, so Firefox doesn't natively block it. If you want to kill it, download the extension AdBlock, then right-click the ad and select it for removal. I believe (don't quote me) you can also kill it by going tools->options->Web Features, clicking "Exceptions" next to "Load Images," and add "http://ad.insightexpress.com/adserver/images/boxquestionmark.gif" to the block list. (Probably a good idea to block "insightexpress.com" all together.)
Personally, I understand the needs for ads on Cnet. However, scripts such as that, the full-page ads that drop down whenever you visit Cnet.com, and other "creative" forms of advertising are simply distasteful. I have my computer set up so I rarely receive ads anywhere, but it irritates me when I do, especially from Cnet. Also, the recent notification from Firefox that 2-6 ad-based tracking cookies are attempting to be installed everytime I open a Cnet newsletter is getting quite annoying. If it's still happening this time next week I'll kick the newsletters out of my inbox along with the cookies. It seems that for the past few weeks Cnet has simply been falling apart (nasty scripts and popups, login problems, and pages not loading/being found). It looks like Lee's going to have his work cut out for him when he gets back...hope he enjoys his vacation while it lasts.
John