Add option in zenLooksAndFeel to enable toolbar-flash-popup

Author: BrhmDev

en-US/browser/browser/preferences/zen-preferences.ftl

@@ -27,6 +27,9 @@ zen-look-and-feel-compact-view-enabled =
 zen-look-and-feel-compact-view-top-toolbar = 
     .label = Hide the top toolbar as well in compact mode
 
+zen-look-and-feel-compact-toolbar-flash-popup =
+    .label = Briefly make the toolbar popup when switching or opening new tabs in compact mode
+
 zen-look-and-feel-buttons-header = Button style
 zen-look-and-feel-buttons-description = Choose the style of buttons you want to see