Ok, i had this too... not with yahoo, but with something called babylon search, which i didnt want AT ALL. I didnt know how to disable it and no one else seemed to know either but i successfully removed it and this is how go to tools (in the task bar), then add ons. go to extentions, or add ons and in there you will find yahoo (hopefully), uninstall and disable it and this should stop it from coming up on your tabs when you open one... This worked for me and now i get blank tabs again :D