--- a/content/base/src/DirectionalityUtils.cpp
+++ b/content/base/src/DirectionalityUtils.cpp
@@ -239,17 +239,21 @@ DoesNotParticipateInAutoDirection(const 
           nodeInfo->Equals(nsGkAtoms::script) ||
           nodeInfo->Equals(nsGkAtoms::style) ||
           nodeInfo->Equals(nsGkAtoms::textarea));
 }
 
 static inline bool
 IsBdiWithoutDirAuto(const Element* aElement)
 {
-  return aElement->IsHTML(nsGkAtoms::bdi) && !aElement->HasDirAuto();
+  // We are testing for bdi elements without explicit dir="auto", so we can't
+  // use the HasDirAuto() flag, since that will return true for bdi element with
+  // no dir attribute or an invalid dir attribute
+  return (aElement->IsHTML(nsGkAtoms::bdi) &&
+          (!aElement->HasValidDir() || aElement->HasFixedDir()));
 }
 
 /**
  * Returns true if aElement is one of the element whose text content should not
  * affect the direction of ancestors with dir=auto (though it may affect its own
  * direction, e.g. <bdi>)
  */
 static bool