--- a/toolkit/components/extensions/ExtensionContent.jsm
+++ b/toolkit/components/extensions/ExtensionContent.jsm
@@ -29,17 +29,17 @@ const DocumentEncoder = Components.Const
   "nsIDocumentEncoder", "init");
 
 const Timer = Components.Constructor("@mozilla.org/timer;1", "nsITimer", "initWithCallback");
 
 ChromeUtils.import("resource://gre/modules/ExtensionChild.jsm");
 ChromeUtils.import("resource://gre/modules/ExtensionCommon.jsm");
 ChromeUtils.import("resource://gre/modules/ExtensionUtils.jsm");
 
-Cu.importGlobalProperties(["crypto", "TextDecoder", "TextEncoder"]);
+Cu.importGlobalProperties(["crypto", "TextEncoder"]);
 
 const {
   DefaultMap,
   DefaultWeakMap,
   defineLazyGetter,
   getInnerWindowID,
   getWinUtils,
   promiseDocumentIdle,
@@ -308,17 +308,17 @@ class Script {
 
   async addCSSCode(cssCode) {
     if (!cssCode) {
       return;
     }
 
     // Store the hash of the cssCode.
     const buffer = await crypto.subtle.digest("SHA-1", new TextEncoder().encode(cssCode));
-    this.cssCodeHash = new TextDecoder().decode(buffer);
+    this.cssCodeHash = String.fromCharCode(...new Uint16Array(buffer));
 
     // Cache and preload the cssCode stylesheet.
     this.cssCodeCache.addCSSCode(this.cssCodeHash, cssCode);
   }
 
   compileScripts() {
     return this.js.map(url => this.scriptCache.get(url));
   }