diff --git a/calendar/experiments/calendar/ext-calendar-utils.sys.mjs b/calendar/experiments/calendar/ext-calendar-utils.sys.mjs
index 3aa1354..d50d9d8 100644
--- a/calendar/experiments/calendar/ext-calendar-utils.sys.mjs
+++ b/calendar/experiments/calendar/ext-calendar-utils.sys.mjs
@@ -239,6 +239,29 @@ export function convertAlarm(item, alarm) {
   };
 }
 
+/**
+ * Thunderbird >=148 no longer exposes cal.createAdapter(). Build explicit
+ * calIObserver objects so the calendar experiment keeps working on newer
+ * release channels as well as ESR.
+ *
+ * @param {object} methods
+ * @returns {calIObserver}
+ */
+export function createCalendarObserver(methods = {}) {
+  return Object.assign({
+    QueryInterface: ChromeUtils.generateQI(["calIObserver"]),
+    onStartBatch() {},
+    onEndBatch() {},
+    onLoad() {},
+    onAddItem() {},
+    onModifyItem() {},
+    onDeleteItem() {},
+    onError() {},
+    onPropertyChanged() {},
+    onPropertyDeleting() {},
+  }, methods);
+}
+
 export async function setupE10sBrowser(extension, browser, parent, initOptions={}) {
   browser.setAttribute("type", "content");
   browser.setAttribute("disableglobalhistory", "true");
diff --git a/calendar/experiments/calendar/parent/ext-calendar-calendars.js b/calendar/experiments/calendar/parent/ext-calendar-calendars.js
index d47a3a4..f6fbd69 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-calendars.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-calendars.js
@@ -13,6 +13,7 @@ this.calendar_calendars = class extends ExtensionAPI {
     const root = `experiments-calendar-${uuid}`;
     const query = context.extension.manifest.version;
     const {
+      createCalendarObserver,
       unwrapCalendar,
       getResolvedCalendarById,
       isOwnCalendar,
@@ -275,7 +276,7 @@ this.calendar_calendars = class extends ExtensionAPI {
             context,
             name: "calendar.calendars.onUpdated",
             register: fire => {
-              const observer = cal.createAdapter(Ci.calIObserver, {
+              const observer = createCalendarObserver({
                 onPropertyChanged(calendar, name, value, _oldValue) {
                   const converted = convertCalendar(context.extension, calendar);
                   switch (name) {
diff --git a/calendar/experiments/calendar/parent/ext-calendar-items.js b/calendar/experiments/calendar/parent/ext-calendar-items.js
index f36b5b0..255ac83 100644
--- a/calendar/experiments/calendar/parent/ext-calendar-items.js
+++ b/calendar/experiments/calendar/parent/ext-calendar-items.js
@@ -13,6 +13,7 @@ this.calendar_items = class extends ExtensionAPI {
     const root = `experiments-calendar-${uuid}`;
     const query = context.extension.manifest.version;
     const {
+      createCalendarObserver,
       getResolvedCalendarById,
       getCachedCalendar,
       isCachedCalendar,
@@ -164,7 +165,7 @@ this.calendar_items = class extends ExtensionAPI {
             context,
             name: "calendar.items.onCreated",
             register: (fire, options) => {
-              const observer = cal.createAdapter(Ci.calIObserver, {
+              const observer = createCalendarObserver({
                 onAddItem: item => {
                   fire.sync(convertItem(item, options, context.extension));
                 },
@@ -181,7 +182,7 @@ this.calendar_items = class extends ExtensionAPI {
             context,
             name: "calendar.items.onUpdated",
             register: (fire, options) => {
-              const observer = cal.createAdapter(Ci.calIObserver, {
+              const observer = createCalendarObserver({
                 onModifyItem: (newItem, _oldItem) => {
                   // TODO calculate changeInfo
                   const changeInfo = {};
@@ -200,7 +201,7 @@ this.calendar_items = class extends ExtensionAPI {
             context,
             name: "calendar.items.onRemoved",
             register: fire => {
-              const observer = cal.createAdapter(Ci.calIObserver, {
+              const observer = createCalendarObserver({
                 onDeleteItem: item => {
                   fire.sync(item.calendar.id, item.id);
                 },