I'm ON Firefox O.o
And since I only get it on this site, it's led to my belief that one of PGC sponsors.
For reference, when this pop-up occurs (actually, it's more like one of those windows asking if you want to install something), the banner at the top of the page is blank except for text that says [object Window], and it seems to be promoting WinFixer 2005 which will "scan my computer for errors for free". It takes about five or six clicks to finally get rid of it.
And no it's not the same thing. This thing keeps asking me to install their stupid program, even when I click "Cancel" and "No", it keeps popping up, saying they're still "scanning". I know it's probably not scanning anything, but it's just damn annoying to deal with this thing every other page.