diff --git a/mfbt/EnumSet.h b/mfbt/EnumSet.h
index ae4364034e266..f635ec51ffbf9 100644
--- a/mfbt/EnumSet.h
+++ b/mfbt/EnumSet.h
@@ -50,8 +50,16 @@ class EnumSet {
     }
   }
 
+#ifdef DEBUG
   constexpr EnumSet(const EnumSet& aEnumSet) : mBitField(aEnumSet.mBitField) {}
 
+  constexpr EnumSet& operator=(const EnumSet& aEnumSet) {
+    mBitField = aEnumSet.mBitField;
+    incVersion();
+    return *this;
+  }
+#endif
+
   /**
    * Add an element
    */
diff --git a/mfbt/tests/TestEnumSet.cpp b/mfbt/tests/TestEnumSet.cpp
index d330f0d4ef6eb..035c1fcfee9ac 100644
--- a/mfbt/tests/TestEnumSet.cpp
+++ b/mfbt/tests/TestEnumSet.cpp
@@ -7,6 +7,8 @@
 #include "mozilla/EnumSet.h"
 #include "mozilla/Vector.h"
 
+#include <type_traits>
+
 using namespace mozilla;
 
 enum SeaBird {
@@ -78,6 +80,8 @@ class EnumSetSuite {
                   "EnumSet should be no bigger than the enum by default");
     static_assert(sizeof(EnumSet<SmallEnum, uint32_t>) == sizeof(uint32_t),
                   "EnumSet should be able to have its size overriden.");
+    static_assert(std::is_trivially_copyable_v<EnumSet<SmallEnum>>,
+                  "EnumSet should be lightweight outside of debug.");
 #endif
   }