userChrome/browser: fix kiosk mode

Make sure the navigation bar is hidden again in kiosk mode. This patch
adds the rest of MR 17 and a comment why it makes sense here.

Fixes: #29

src/userChrome/browser.css

@@ -6,4 +6,13 @@
     #browser {
         -moz-box-ordinal-group: 0 !important;
     }
+
+    /* Hide navigation bar in kiosk mode (to prevent bug #29). We can assume FF
+     * is in kiosk mode when fullscreen and max-width conditions are met,
+     * because at this max-width the fullscreen button is hidden
+     * (see appMenu.css). */
+    #nav-bar[inFullscreen],
+    #TabsToolbar[inFullscreen] {
+        display: none;
+    }
 }