--- a/toolkit/components/extensions/ExtensionParent.jsm
+++ b/toolkit/components/extensions/ExtensionParent.jsm
@@ -90,41 +90,34 @@ let apiManager = new class extends Schem
   }
 
   // Loads all the ext-*.js scripts currently registered.
   lazyInit() {
     if (this.initialized) {
       return this.initialized;
     }
 
-    let scripts = [];
     for (let [/* name */, value] of XPCOMUtils.enumerateCategoryEntries(CATEGORY_EXTENSION_SCRIPTS)) {
-      scripts.push(value);
+      Services.scriptloader.loadSubScript(value, this.global);
     }
 
-    let promise = Promise.all(scripts.map(url => ChromeUtils.compileScript(url))).then(scripts => {
-      for (let script of scripts) {
-        script.executeInGlobal(this.global);
+    // Load order matters here. The base manifest defines types which are
+    // extended by other schemas, so needs to be loaded first.
+    let promise = Schemas.load(BASE_SCHEMA).then(() => {
+      let promises = [];
+      for (let [/* name */, url] of XPCOMUtils.enumerateCategoryEntries(CATEGORY_EXTENSION_SCHEMAS)) {
+        promises.push(Schemas.load(url));
       }
-
-      // Load order matters here. The base manifest defines types which are
-      // extended by other schemas, so needs to be loaded first.
-      return Schemas.load(BASE_SCHEMA).then(() => {
-        let promises = [];
-        for (let [/* name */, url] of XPCOMUtils.enumerateCategoryEntries(CATEGORY_EXTENSION_SCHEMAS)) {
-          promises.push(Schemas.load(url));
-        }
-        for (let url of this.schemaURLs) {
-          promises.push(Schemas.load(url));
-        }
-        for (let url of schemaURLs) {
-          promises.push(Schemas.load(url));
-        }
-        return Promise.all(promises);
-      });
+      for (let url of this.schemaURLs) {
+        promises.push(Schemas.load(url));
+      }
+      for (let url of schemaURLs) {
+        promises.push(Schemas.load(url));
+      }
+      return Promise.all(promises);
     });
 
     /* eslint-disable mozilla/balanced-listeners */
     Services.mm.addMessageListener("Extension:GetTabAndWindowId", this);
     /* eslint-enable mozilla/balanced-listeners */
 
     this.initialized = promise;
     return this.initialized;