Which desktop are you using, KDE, Mate, Cinnamon, Xfce, Unity? Did you install over the Unity desktop or are you presently using Unity? This sounds like you might possibly be getting the Unity menu response, or if it only happens in Firefox, see the next paragraph.
Do you have an orange rectangle with firefox in the line with your tabs? You may have changed to the minimal menu on Firefox. If that is the case, click on the Firefox to open the menu >> click Preferences >> and tick the box by the Menu Bar.