How can I get rid of this annoying pop up window from Facebook-or is it a virus??

I keep getting random "Facebook Password " requests via a pop-up window, asking me to enter the password for facebook. It has to button options Cancel & Preferences (which opens the preferences pane. If I cancel the window it re-appears eight times before disappearing for about five minutes then reappears again.

I have removed all unknown browser extensions and it's still happening. very annoying!