Firefox not restoring both windows

I always have two windows of Firefox open, one for each monitor. Up until recently Firefox would always restore both windows exactly as they were before I right clicked the Firefox icon in the taskbar and clicked "close all windows". This week however my browser will only restore one of the two windows I had open, and I have to restore the other one manually which is annoying. Can anyone help me bring back the old behavior so it restores both my windows every time?