Over the last week or so I've had new tabs open in Firefox. Firefox blocks them with the "Reported Web Forgery!" warning. I've had some for "new browser version" and one for an over 18s site. As far as I am aware my PC is clean. They occur during normal web browsing. I visit a lot of discussion forums and I think they've all popped up whilst using Zetaboards or Invisionfree forums. What could be causing these new tabs to open?