--- a/dom/base/nsContentPermissionHelper.h
+++ b/dom/base/nsContentPermissionHelper.h
@@ -131,26 +131,34 @@ class ContentPermissionRequestBase : pub
   enum class DelayedTaskType {
     Allow,
     Deny,
     Request,
   };
   void RequestDelayedTask(nsIEventTarget* aTarget, DelayedTaskType aType);
 
  protected:
+  // @param aPrefName see `mPrefName`.
   ContentPermissionRequestBase(nsIPrincipal* aPrincipal,
                                nsPIDOMWindowInner* aWindow,
                                const nsACString& aPrefName,
                                const nsACString& aType);
   virtual ~ContentPermissionRequestBase() = default;
 
   nsCOMPtr<nsIPrincipal> mPrincipal;
   nsCOMPtr<nsIPrincipal> mTopLevelPrincipal;
   nsCOMPtr<nsPIDOMWindowInner> mWindow;
   RefPtr<PermissionDelegateHandler> mPermissionHandler;
+
+  // The prefix of a pref which allows tests to bypass showing the prompt.
+  // Tests will have to set both of
+  // ${mPrefName}.prompt.testing and
+  // ${mPrefName}.prompt.testing.allow
+  // to either true or false. If no such testing is required, mPrefName may be
+  // empty.
   const nsCString mPrefName;
   const nsCString mType;
   bool mIsHandlingUserInput;
   bool mMaybeUnsafePermissionDelegate;
 };
 
 }  // namespace dom
 }  // namespace mozilla