--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -994,34 +994,32 @@ void CustomElementRegistry::Define(
     disableInternals = disabledFeatures.Contains(
         static_cast<nsStaticAtom*>(nsGkAtoms::internals));
 
     // 14.10. Set disableShadow to true if disabledFeaturesSequence contains
     //        "shadow".
     disableShadow = disabledFeatures.Contains(
         static_cast<nsStaticAtom*>(nsGkAtoms::shadow));
 
-    if (StaticPrefs::dom_webcomponents_formAssociatedCustomElement_enabled()) {
-      // 14.11. Let formAssociatedValue be Get(constructor, "formAssociated").
-      //        Rethrow any exceptions.
-      JS::Rooted<JS::Value> formAssociatedValue(aCx);
-      if (!JS_GetProperty(aCx, constructor, "formAssociated",
-                          &formAssociatedValue)) {
-        aRv.NoteJSContextException(aCx);
-        return;
-      }
+    // 14.11. Let formAssociatedValue be Get(constructor, "formAssociated").
+    //        Rethrow any exceptions.
+    JS::Rooted<JS::Value> formAssociatedValue(aCx);
+    if (!JS_GetProperty(aCx, constructor, "formAssociated",
+                        &formAssociatedValue)) {
+      aRv.NoteJSContextException(aCx);
+      return;
+    }
 
-      // 14.12. Set formAssociated to the result of converting
-      //        formAssociatedValue to a boolean. Rethrow any exceptions from
-      //        the conversion.
-      if (!ValueToPrimitive<bool, eDefault>(
-              aCx, formAssociatedValue, "formAssociated", &formAssociated)) {
-        aRv.NoteJSContextException(aCx);
-        return;
-      }
+    // 14.12. Set formAssociated to the result of converting
+    //        formAssociatedValue to a boolean. Rethrow any exceptions from
+    //        the conversion.
+    if (!ValueToPrimitive<bool, eDefault>(aCx, formAssociatedValue,
+                                          "formAssociated", &formAssociated)) {
+      aRv.NoteJSContextException(aCx);
+      return;
     }
   }  // Unset mIsCustomDefinitionRunning
 
   /**
    * 15. Let definition be a new custom element definition with name name,
    *     local name localName, constructor constructor, prototype prototype,
    *     observed attributes observedAttributes, and lifecycle callbacks
    *     lifecycleCallbacks.

--- a/dom/html/HTMLElement.cpp
+++ b/dom/html/HTMLElement.cpp
@@ -263,21 +263,17 @@ void HTMLElement::UpdateFormOwner(bool a
     args.mForm = newForm;
     nsContentUtils::EnqueueLifecycleCallback(
         ElementCallbackType::eFormAssociated, this, args);
   }
 }
 
 bool HTMLElement::IsFormAssociatedElement() const {
   CustomElementData* data = GetCustomElementData();
-  bool isFormAssociatedCustomElement = data && data->IsFormAssociated();
-  MOZ_ASSERT_IF(
-      isFormAssociatedCustomElement,
-      StaticPrefs::dom_webcomponents_formAssociatedCustomElement_enabled());
-  return isFormAssociatedCustomElement;
+  return data && data->IsFormAssociated();
 }
 
 void HTMLElement::FieldSetDisabledChanged(bool aNotify) {
   // This *has* to be called *before* UpdateBarredFromConstraintValidation
   // because this function depend on our disabled state.
   nsGenericHTMLFormElement::FieldSetDisabledChanged(aNotify);
 
   UpdateBarredFromConstraintValidation();

--- a/dom/tests/browser/browser_form_associated_custom_elements_validity.js
+++ b/dom/tests/browser/browser_form_associated_custom_elements_validity.js
@@ -1,21 +1,15 @@
 /**
  * Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 "use strict";
 
-add_task(async function init() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["dom.webcomponents.formAssociatedCustomElement.enabled", true]],
-  });
-});
-
 add_task(async function report_validity() {
   await BrowserTestUtils.withNewTab(
     {
       gBrowser,
       url: `data:text/html,<my-control></my-control>`,
     },
     async function(aBrowser) {
       let promisePopupShown = BrowserTestUtils.waitForEvent(

--- a/dom/webidl/ElementInternals.webidl
+++ b/dom/webidl/ElementInternals.webidl
@@ -8,39 +8,39 @@
  */
 
 [Exposed=Window]
 interface ElementInternals {
   // Shadow root access
   readonly attribute ShadowRoot? shadowRoot;
 
   // Form-associated custom elements
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   void setFormValue((File or USVString or FormData)? value,
                     optional (File or USVString or FormData)? state);
 
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   readonly attribute HTMLFormElement? form;
 
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   void setValidity(optional ValidityStateFlags flags = {},
                    optional DOMString message,
                    optional HTMLElement anchor);
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   readonly attribute boolean willValidate;
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   readonly attribute ValidityState validity;
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   readonly attribute DOMString validationMessage;
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   boolean checkValidity();
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   boolean reportValidity();
 
-  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  [Throws]
   readonly attribute NodeList labels;
 };
 
 partial interface ElementInternals {
   [ChromeOnly, Throws]
   readonly attribute HTMLElement? validationAnchor;
 };
 

--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -4103,22 +4103,16 @@
 
 # NOTE: This preference is used in unit tests. If it is removed or its default
 # value changes, please update test_sharedMap_static_prefs.js accordingly.
 - name: dom.webcomponents.shadowdom.report_usage
   type: bool
   value: false
   mirror: always
 
-# Is support for form-associated custom element enabled?
-- name: dom.webcomponents.formAssociatedCustomElement.enabled
-  type: bool
-  value: true
-  mirror: always
-
 # Is support for the Web GPU API enabled?
 - name: dom.webgpu.enabled
   type: RelaxedAtomicBool
   value: false
   mirror: always
 
 # Is support for HTMLInputElement.webkitEntries enabled?
 - name: dom.webkitBlink.filesystem.enabled

--- a/testing/web-platform/meta/custom-elements/__dir__.ini
+++ b/testing/web-platform/meta/custom-elements/__dir__.ini
@@ -1,2 +1,1 @@
-prefs: [dom.webcomponents.formAssociatedCustomElement.enabled:true]
 leak-threshold: [default:51200]