Due to some malware, I think, because something like that used to appear on every webpage I open, but I don't remember how I got rid of them anymore. By deleting something, maybe.
BTW I'm now harassed by something worse (random words on every page are green, double underlined and and whenever my mouse points to it something like "you could win an iPad" pops up) everyday, and I don't know what to do with it. Maybe I didn't get rid of them but they changed forms recently?