Not a bug
Firefox keeps being moved to the right-end of the taskbar automatically only when 7+ is open, I always have it as the second from the left. 7+ has a bug (or a "feature") that keeps moving/auto-arranging window taskbar button positions for no clear/intuitive/useful reason. The only reason I'm using 7+ is to forcibly ungroup window taskbar buttons, this bug is so severe I'm thinking of going back to 64 bit XP.
Customer support service by UserEcho