diff --git a/web/firefoxcom.js b/web/firefoxcom.js
index 6d8b35250..8673a3fd0 100644
--- a/web/firefoxcom.js
+++ b/web/firefoxcom.js
@@ -174,13 +174,9 @@ class DownloadManager {
 }
 
 class FirefoxPreferences extends BasePreferences {
-  async _writeToStorage(prefObj) {
-    return FirefoxCom.requestAsync("setPreferences", prefObj);
-  }
-
   async _readFromStorage(prefObj) {
-    const prefStr = await FirefoxCom.requestAsync("getPreferences", prefObj);
-    return JSON.parse(prefStr);
+    const prefs = await FirefoxCom.requestAsync("getPreferences", prefObj);
+    return typeof prefs === "string" ? JSON.parse(prefs) : prefs;
   }
 }
 
diff --git a/web/preferences.js b/web/preferences.js
index eebc480ca..dead99e98 100644
--- a/web/preferences.js
+++ b/web/preferences.js
@@ -83,6 +83,9 @@ class BasePreferences {
    *                    have been reset.
    */
   async reset() {
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Please use `about:config` to change preferences.");
+    }
     await this.#initializedPromise;
     const prefs = this.#prefs;
 
@@ -102,6 +105,9 @@ class BasePreferences {
    *                    provided that the preference exists and the types match.
    */
   async set(name, value) {
+    if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Please use `about:config` to change preferences.");
+    }
     await this.#initializedPromise;
     const defaultValue = this.#defaults[name],
       prefs = this.#prefs;