I did some poking around and found the cause, and how to fix it. It IS a notice from the Rosetta project, but it isn't supposed to open the video in a web browser automatically. It's actually BoincTasks that is loading the video.
Inside BoincTasks, click on Extra, then click the Notices tab. Uncheck the "Show alert every" box, and change "Check for new notices" to 0 minutes. That will make it so notices don't automatically load/pop up messages.
Apparently BoincManager does the same thing, but I'm not sure how to turn it off since I don't have that one installed.