Anyone have insight into this Yahoo mail browser popup alert?

Alert
altfarm.mediaplex.com has sent an incorrect or unexpected message.
Error Code: -12263

I get this every day with Firefox on WinXP in my yahoo email.

I googled but mediaplex doesn't seem to be out there much. Why would just I
get this yahoo-related error message?

I do use a huge hosts file I got from the internet that blocks all sorts of
media stuff.

Here are the mediaplex lines:
127.0.0.1 mediaplex.com #[eTrust.Tracking Cookie]
127.0.0.1 adfarm.mediaplex.com #[Adware.Meplex][Adware-DesktopMedia.dr]
127.0.0.1 altfarm.mediaplex.com #[may affect eBay][MVPS.Criteria]
127.0.0.1 img.mediaplex.com #[Ewido.TrackingCookie.Mediaplex]
127.0.0.1 img-snv.mediaplex.com #[Tenebril.Tracking Cookie]
127.0.0.1 www.mediaplex.com #[SunBelt.Mediaplex.com]

What is this mediaplex thingey trying to do to me?