diff --git a/files/en-us/mozilla/firefox/releases/139/index.md b/files/en-us/mozilla/firefox/releases/139/index.md
index 94b0656ef9a2e8e..ef94bc2ca19e034 100644
--- a/files/en-us/mozilla/firefox/releases/139/index.md
+++ b/files/en-us/mozilla/firefox/releases/139/index.md
@@ -57,6 +57,8 @@ This article provides information about the changes in Firefox 139 that affect d
 
 ## Changes for add-on developers
 
+- Localized extensions now cascade through locale subtags to find translations before reverting to the extension's default language. Previously, the extension used the extension default if a translation couldn't be found for a language with subtags. See [Localized string selection](/en-US/docs/Mozilla/Add-ons/WebExtensions/Internationalization#localized_string_selection) in the Internationalization article for more details of the new behavior. ([Firefox bug 1381580](https://bugzil.la/1381580))
+
 ### Removals
 
 ### Other