diff --git a/gfx/layers/LayerMetricsWrapper.h b/gfx/layers/LayerMetricsWrapper.h
index b390b5eda95fd..ee1c0655156ac 100644
--- a/gfx/layers/LayerMetricsWrapper.h
+++ b/gfx/layers/LayerMetricsWrapper.h
@@ -168,7 +168,7 @@ class MOZ_STACK_CLASS LayerMetricsWrapper {
     return mLayer != nullptr;
   }
 
-  MOZ_EXPLICIT_CONVERSION operator bool() const
+  explicit operator bool() const
   {
     return IsValid();
   }
diff --git a/mfbt/Attributes.h b/mfbt/Attributes.h
index ce536038ca573..f1767272343d6 100644
--- a/mfbt/Attributes.h
+++ b/mfbt/Attributes.h
@@ -55,13 +55,10 @@
 #  define MOZ_HAVE_NEVER_INLINE          __declspec(noinline)
 #  define MOZ_HAVE_NORETURN              __declspec(noreturn)
 #  ifdef __clang__
-     /* clang-cl probably supports constexpr and explicit conversions. */
+     /* clang-cl probably supports constexpr. */
 #    if __has_extension(cxx_constexpr)
 #      define MOZ_HAVE_CXX11_CONSTEXPR
 #    endif
-#    if __has_extension(cxx_explicit_conversions)
-#      define MOZ_HAVE_EXPLICIT_CONVERSION
-#    endif
 #  endif
 #elif defined(__clang__)
    /*
@@ -75,9 +72,6 @@
 #  if __has_extension(cxx_constexpr)
 #    define MOZ_HAVE_CXX11_CONSTEXPR
 #  endif
-#  if __has_extension(cxx_explicit_conversions)
-#    define MOZ_HAVE_EXPLICIT_CONVERSION
-#  endif
 #  if __has_extension(cxx_override_control)
 #    define MOZ_HAVE_CXX11_OVERRIDE
 #    define MOZ_HAVE_CXX11_FINAL         final
@@ -97,9 +91,6 @@
 #    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 0)
 #      define MOZ_HAVE_CXX11_CONSTEXPR
 #    endif
-#    if MOZ_GCC_VERSION_AT_LEAST(4, 5, 0)
-#      define MOZ_HAVE_EXPLICIT_CONVERSION
-#    endif
 #  else
      /* __final is a non-C++11 GCC synonym for 'final', per GCC r176655. */
 #    if MOZ_GCC_VERSION_AT_LEAST(4, 7, 0)
@@ -138,31 +129,6 @@
 #  define MOZ_CONSTEXPR_VAR     const
 #endif
 
-/*
- * MOZ_EXPLICIT_CONVERSION is a specifier on a type conversion
- * overloaded operator that declares that a C++11 compiler should restrict
- * this operator to allow only explicit type conversions, disallowing
- * implicit conversions.
- *
- * Example:
- *
- *   template<typename T>
- *   class Ptr
- *   {
- *     T* mPtr;
- *     MOZ_EXPLICIT_CONVERSION operator bool() const
- *     {
- *       return mPtr != nullptr;
- *     }
- *   };
- *
- */
-#ifdef MOZ_HAVE_EXPLICIT_CONVERSION
-#  define MOZ_EXPLICIT_CONVERSION explicit
-#else
-#  define MOZ_EXPLICIT_CONVERSION /* no support */
-#endif
-
 /*
  * MOZ_NEVER_INLINE is a macro which expands to tell the compiler that the
  * method decorated with it must never be inlined, even if the compiler would
diff --git a/mfbt/TypedEnumInternal.h b/mfbt/TypedEnumInternal.h
index 5af19e20202ec..4ef0767650896 100644
--- a/mfbt/TypedEnumInternal.h
+++ b/mfbt/TypedEnumInternal.h
@@ -81,7 +81,7 @@ class CastableTypedEnumResult
   MOZ_CONSTEXPR operator E() const { return mValue; }
 
   template<typename DestinationType>
-  MOZ_EXPLICIT_CONVERSION MOZ_CONSTEXPR
+  explicit MOZ_CONSTEXPR
   operator DestinationType() const { return DestinationType(mValue); }
 
   MOZ_CONSTEXPR bool operator !() const { return !bool(mValue); }
diff --git a/mfbt/tests/TestTypedEnum.cpp b/mfbt/tests/TestTypedEnum.cpp
index f08024325aeba..e9f39bf9bba20 100644
--- a/mfbt/tests/TestTypedEnum.cpp
+++ b/mfbt/tests/TestTypedEnum.cpp
@@ -160,7 +160,7 @@ TestNonConvertibilityForOneType()
 {
   using mozilla::IsConvertible;
 
-#if defined(MOZ_HAVE_CXX11_STRONG_ENUMS) && defined(MOZ_HAVE_EXPLICIT_CONVERSION)
+#if defined(MOZ_HAVE_CXX11_STRONG_ENUMS)
   static_assert(!IsConvertible<T, bool>::value, "should not be convertible");
   static_assert(!IsConvertible<T, int>::value, "should not be convertible");
   static_assert(!IsConvertible<T, uint64_t>::value, "should not be convertible");
@@ -435,18 +435,12 @@ void TestNoConversionsBetweenUnrelatedTypes()
   static_assert(!IsConvertible<decltype(T1::A), decltype(T2::A | T2::B)>::value,
                 "should not be convertible");
 
-  // The following are #ifdef MOZ_HAVE_EXPLICIT_CONVERSION because without
-  // support for explicit conversion operators, we can't easily have these bad
-  // conversions completely removed. They still do fail to compile in practice,
-  // but not in a way that we can static_assert on.
-#ifdef MOZ_HAVE_EXPLICIT_CONVERSION
   static_assert(!IsConvertible<decltype(T1::A | T1::B), T2>::value,
                 "should not be convertible");
   static_assert(!IsConvertible<decltype(T1::A | T1::B), decltype(T2::A)>::value,
                 "should not be convertible");
   static_assert(!IsConvertible<decltype(T1::A | T1::B), decltype(T2::A | T2::B)>::value,
                 "should not be convertible");
-#endif
 }
 
 MOZ_BEGIN_ENUM_CLASS(Int8EnumWithHighBits, int8_t)
diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index f55ce9bfafdfd..fd08acd6549ac 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -878,13 +878,13 @@ class WidgetGUIEvent : public WidgetEvent
 
   public:
 
-    MOZ_EXPLICIT_CONVERSION operator bool() const
+    explicit operator bool() const
     {
       return !mBuffer.IsEmpty();
     }
 
     template<typename T>
-    MOZ_EXPLICIT_CONVERSION operator const T*() const
+    explicit operator const T*() const
     {
       return mBuffer.IsEmpty()
              ? nullptr