This recently happened to me. I was using Mozilla Firefox and Yahoo showed up in an unwelcome fashion. My home computer is used by my wife, whose computer skills are not great, and by two children who play online games constantly, so it is conceivable that Yahoo was given "permission" by somebody. Long story short, I made sure that Explorer was working, and deleted Firefox and re-installed.
There is nothing more infuriating than every time you type in a URL, it is treated as a search, with no direct entry option.
Yahoo has been around the block, and they have to know that they have gotten into the realm of malware. The [censored] with them -- what's next from Yahoo? The Nigerian letter scams?