diff --git a/components/style/values/specified/box.rs b/components/style/values/specified/box.rs
index 47561c9a51a..660b77b32e8 100644
--- a/components/style/values/specified/box.rs
+++ b/components/style/values/specified/box.rs
@@ -300,6 +300,8 @@ impl Display {
             DisplayInside::Flex => true,
             #[cfg(feature = "gecko")]
             DisplayInside::Grid => true,
+            #[cfg(feature = "gecko")]
+            DisplayInside::MozBox => true,
             _ => false,
         }
     }
@@ -337,7 +339,7 @@ impl Display {
                 };
                 Display::from3(DisplayOutside::Block, inside, self.is_list_item())
             },
-            DisplayOutside::Block | DisplayOutside::None => *self,
+            DisplayOutside::Block | DisplayOutside::XUL | DisplayOutside::None => *self,
             _ => Display::Block,
         }
     }