My drop-down menu box on Firefox recently switched from most frequently visited sites at the top to a random order. How can I change it back so that the sites I visit most often comes up first?

Before the change, I would click on the downward arrow next to the address box and a list of the 10 sites I visit most frequently would drop down. Mozilla was always first, then facebook, and so on. Now, mozilla is in the middle, facebook is 4th down, and the site I visit least often is at the top. I didn't change anything in my settings, though mozilla did update recently. Any suggestions?