--- a/modules/libpref/Preferences.cpp
+++ b/modules/libpref/Preferences.cpp
@@ -3151,16 +3151,20 @@ struct CacheData
     uint32_t mDefaultValueUint;
     float mDefaultValueFloat;
   };
 };
 
 // gCacheDataDesc holds information about prefs startup. It's being used for
 // diagnosing prefs startup problems in bug 1276488.
 static const char* gCacheDataDesc = "untouched";
+
+// gCacheData holds the CacheData objects used for VarCache prefs. It owns
+// those objects, and also is used to detect if multiple VarCaches get tied to
+// a single global variable.
 static nsTArray<nsAutoPtr<CacheData>>* gCacheData = nullptr;
 
 #ifdef DEBUG
 static bool
 HaveExistingCacheFor(void* aPtr)
 {
   MOZ_ASSERT(NS_IsMainThread());
   if (gCacheData) {