This is a weird one. On MediaFire downloads, there's a checkbox under the download link and unless you uncheck it before you click download, it will open a popup. You may have to refresh the page a few times, because the checkbox isn't there every time. Is it possible to block this?
the only popup i see is on the bottom asking you to Upgrade to Mediafire Pro account so you get such and such features, etc...
if you want you can add the following DIV to hide it
Here's a screenshot of the ad.
I got it on this page. http://www.mediafire.com/?mtnghymn4cc
The checkbox appears under the download link and opens a popup when you click the link unless you uncheck the box. I think it's that pmsrvr.com thing.