diff --git a/mfbt/WeakPtr.h b/mfbt/WeakPtr.h
index ebcb9250674..58960ad6177 100644
--- a/mfbt/WeakPtr.h
+++ b/mfbt/WeakPtr.h
@@ -183,7 +183,7 @@ public:
     return *this = aOther->SelfReferencingWeakPtr();
   }
 
-  WeakPtr(T* aOther)
+  MOZ_IMPLICIT WeakPtr(T* aOther)
   {
     *this = aOther;
   }
diff --git a/mfbt/tests/TestPair.cpp b/mfbt/tests/TestPair.cpp
index 37aa2c69410..940174df0f3 100644
--- a/mfbt/tests/TestPair.cpp
+++ b/mfbt/tests/TestPair.cpp
@@ -29,21 +29,21 @@ using mozilla::Pair;
 INSTANTIATE(int, int, prim1, 2 * sizeof(int));
 INSTANTIATE(int, long, prim2, 2 * sizeof(long));
 
-struct EmptyClass { EmptyClass(int) {} };
-struct NonEmpty { char mC; NonEmpty(int) {} };
+struct EmptyClass { explicit EmptyClass(int) {} };
+struct NonEmpty { char mC; explicit NonEmpty(int) {} };
 
 INSTANTIATE(int, EmptyClass, both1, sizeof(int));
 INSTANTIATE(int, NonEmpty, both2, 2 * sizeof(int));
 INSTANTIATE(EmptyClass, NonEmpty, both3, 1);
 
-struct A { char dummy; A(int) {} };
-struct B : A { B(int aI) : A(aI) {} };
+struct A { char dummy; explicit A(int) {} };
+struct B : A { explicit B(int aI) : A(aI) {} };
 
 INSTANTIATE(A, A, class1, 2);
 INSTANTIATE(A, B, class2, 2);
 INSTANTIATE(A, EmptyClass, class3, 1);
 
-struct OtherEmpty : EmptyClass { OtherEmpty(int aI) : EmptyClass(aI) {} };
+struct OtherEmpty : EmptyClass { explicit OtherEmpty(int aI) : EmptyClass(aI) {} };
 
 // C++11 requires distinct objects of the same type, within the same "most
 // derived object", to have different addresses.  Pair allocates its elements as
diff --git a/mfbt/tests/TestSplayTree.cpp b/mfbt/tests/TestSplayTree.cpp
index b49371f3237..e1511e1abe0 100644
--- a/mfbt/tests/TestSplayTree.cpp
+++ b/mfbt/tests/TestSplayTree.cpp
@@ -89,7 +89,7 @@ static int gValues[] = {
 
 struct SplayInt : SplayTreeNode<SplayInt>
 {
-  SplayInt(int aValue) : mValue(aValue) {}
+  explicit SplayInt(int aValue) : mValue(aValue) {}
 
   static int compare(const SplayInt& aOne, const SplayInt& aTwo)
   {