diff --git a/dom/base/nsDeprecatedOperationList.h b/dom/base/nsDeprecatedOperationList.h
index 885f25244ceb..aad87ba8aa9d 100644
--- a/dom/base/nsDeprecatedOperationList.h
+++ b/dom/base/nsDeprecatedOperationList.h
@@ -47,3 +47,4 @@ DEPRECATED_OPERATION(AppCache)
 DEPRECATED_OPERATION(PrefixedFullscreenAPI)
 DEPRECATED_OPERATION(LenientSetter)
 DEPRECATED_OPERATION(NavigatorBattery)
+DEPRECATED_OPERATION(FileLastModifiedDate)
diff --git a/dom/locales/en-US/chrome/dom/dom.properties b/dom/locales/en-US/chrome/dom/dom.properties
index b4dd3faed132..e8e0c1c00421 100644
--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -229,3 +229,4 @@ PushMessageDecryptionFailure=The ServiceWorker for scope ‘%1$S’ encountered
 # LOCALIZATION NOTE: %1$S is the type of a DOM event. 'passive' is a literal parameter from the DOM spec.
 PreventDefaultFromPassiveListenerWarning=Ignoring ‘preventDefault()’ call on event of type ‘%1$S’ from a listener registered as ‘passive’.
 NavigatorBatteryWarning=navigator.battery is deprecated. Use navigator.getBattery() instead.
+FileLastModifiedDateWarning=File.lastModifiedDate is deprecated. Use File.lastModified instead.
diff --git a/dom/webidl/File.webidl b/dom/webidl/File.webidl
index a6b42c70c9d4..31f0aecf2923 100644
--- a/dom/webidl/File.webidl
+++ b/dom/webidl/File.webidl
@@ -41,7 +41,7 @@ dictionary ChromeFilePropertyBag : FilePropertyBag {
 // Mozilla extensions
 partial interface File {
 
-  [GetterThrows]
+  [GetterThrows, Deprecated="FileLastModifiedDate"]
   readonly attribute Date lastModifiedDate;
 
   [BinaryName="path", Func="mozilla::dom::Directory::WebkitBlinkDirectoryPickerEnabled"]