Firefox tab problem

Hey so while scrolling down through a webpage in firefox on 1 of my tabs it will sometimes open a new window and put the tab I was viewing in that new firefox window. I am not sure if I am clicking something by accident to make it do this. After this happens I then go to the new window and have to copy that tab back to my original firefox window which is getting really annyoing. I think that the command is the one called open in a new window when you right click a tab but I never do this. So anyone have any ideas?