diff --git a/l10n/en-US/viewer.properties b/l10n/en-US/viewer.properties
index e66fa1046e0db..f64be7bf226c3 100644
--- a/l10n/en-US/viewer.properties
+++ b/l10n/en-US/viewer.properties
@@ -43,7 +43,9 @@ save.title=Save
 save_label=Save
 bookmark1.title=Current Page (View URL from Current Page)
 bookmark1_label=Current Page
+# LOCALIZATION NOTE (open_in_app.title): This string is used in Firefox for Android.
 open_in_app.title=Open in app
+# LOCALIZATION NOTE (open_in_app_label): This string is used in Firefox for Android. Length of the translation matters since we are in a mobile context, with limited screen estate.
 open_in_app_label=Open in app
 
 # Secondary toolbar and context menu