I assume it's malware, at least. The only programs I've installed recently are CutePDF and the Dungeons and Dragons Character Builder. I did a search for spyware on both of them (at least, the best I could), but nothing conclusive. When I do a google search, every result is flagged "This site may harm your computer", and I get the interstitial page from Google when I click on it. But the links on the interstitial page all time out when I try to get more info. Even www.stopbadware.org
Edit: Okay, now I get this instead:
Your client does not have permission to get URL /interstitial?url=http://www.wizards.com/default.asp%3Fx=dnd/insider/characterbuilder
from this server. (Client IP address: 192.xxx.xxx.xxx)
I downloaded and ran the newest version of spybot, but it didn't help. Windows Defender hasn't picked up anything, and neither has Avast!, both of which are up to date.
Edit: And it's only in Firefox. I've only got a handful of benign add-ons that I've been using for a long time.