diff --git a/scripts/migrate-fx140/is-esmified.js b/scripts/migrate-fx140/is-esmified.js
index 872b4cd877..d3bb1fc13b 100755
--- a/scripts/migrate-fx140/is-esmified.js
+++ b/scripts/migrate-fx140/is-esmified.js
@@ -33,6 +33,10 @@ function generateESMURIMap(jsm_map) {
 }
 
 function isESMifiedSlow(resourceURI) {
+  if (!resourceURI.includes('/zotero/') && resourceURI.endsWith('.jsm')) {
+    return { result: true, jsms: [resourceURI.replace('.jsm', '.sys.mjs')] };
+  }
+  
   if (!(resourceURI in uri_map)) {
     console.log(`WARNING: Unknown module: ${resourceURI}`);
     return { result: false, jsms: [] };