diff --git a/webextensions/background/background-cache.js b/webextensions/background/background-cache.js
index e8f054a40..bac687be6 100644
--- a/webextensions/background/background-cache.js
+++ b/webextensions/background/background-cache.js
@@ -18,6 +18,7 @@ import * as MetricsData from '/common/metrics-data.js';
 import * as TabsInternalOperation from '/common/tabs-internal-operation.js';
 import * as TabsStore from '/common/tabs-store.js';
 import * as TabsUpdate from '/common/tabs-update.js';
+import * as UniqueId from '/common/unique-id.js';
 
 import Tab from '/common/Tab.js';
 
@@ -300,7 +301,7 @@ async function updateWindowCache(owner, key, value) {
 
   if (!configs.persistCachedTree &&
       browser.storage.session) {
-    const storagKey = `backgroundCache-window${owner.windowId}-${key}`;
+    const storagKey = `backgroundCache-${await UniqueId.ensureWindowId(owner.windowId)}-${key}`;
     if (value) {
       const data = {};
       data[storagKey] = value;
@@ -343,7 +344,7 @@ export function markWindowCacheDirtyFromTab(tab, akey) {
 
 async function getWindowCache(owner, key) {
   if (!configs.persistCachedTree) {
-    const storageKey = `backgroundCache-window${owner.windowId}-${key}`;
+    const storageKey = `backgroundCache-${await UniqueId.ensureWindowId(owner.windowId)}-${key}`;
     const defaultData = {};
     defaultData[storageKey] = undefined;
     return browser.storage.session.get(defaultData).then(data => {
diff --git a/webextensions/common/unique-id.js b/webextensions/common/unique-id.js
index b0861e99c..89969cbe9 100644
--- a/webextensions/common/unique-id.js
+++ b/webextensions/common/unique-id.js
@@ -151,17 +151,31 @@ export async function request(tabOrId, options = {}) {
     }
   }
 
-  const adjective   = kID_ADJECTIVES[Math.floor(Math.random() * kID_ADJECTIVES.length)];
-  const noun        = kID_NOUNS[Math.floor(Math.random() * kID_NOUNS.length)];
-  const randomValue = Math.floor(Math.random() * 1000);
-  const id          = `tab-${adjective}-${noun}-${Date.now()}-${randomValue}`;
+  const id = `tab-${generate()}`;
   // tabId is for detecttion of duplicated tabs
   await browser.sessions.setTabValue(tab.id, Constants.kPERSISTENT_ID, { id, tabId: tab.id }).catch(ApiTabs.createErrorSuppressor());
   return { id, originalId, originalTabId, duplicated };
 }
 
+function generate() {
+  const adjective   = kID_ADJECTIVES[Math.floor(Math.random() * kID_ADJECTIVES.length)];
+  const noun        = kID_NOUNS[Math.floor(Math.random() * kID_NOUNS.length)];
+  const randomValue = Math.floor(Math.random() * 1000);
+  return `${adjective}-${noun}-${Date.now()}-${randomValue}`;
+}
+
 export async function getFromTabs(tabs) {
   return Promise.all(tabs.map(tab =>
     browser.sessions.getTabValue(tab.id, Constants.kPERSISTENT_ID).catch(ApiTabs.createErrorHandler())
   ));
 }
+
+export async function ensureWindowId(windowId) {
+  const storedUniqueId = await browser.sessions.getWindowValue(windowId, 'uniqueId').catch(_ => null);
+  if (storedUniqueId)
+    return storedUniqueId;
+
+  const uniqueId = `window-${generate()}`;
+  await browser.sessions.setWindowValue(windowId, 'uniqueId', uniqueId).catch(_ => null);
+  return uniqueId;
+}
diff --git a/webextensions/sidebar/sidebar-cache.js b/webextensions/sidebar/sidebar-cache.js
index 90cfe7ec9..f5ab172a4 100644
--- a/webextensions/sidebar/sidebar-cache.js
+++ b/webextensions/sidebar/sidebar-cache.js
@@ -21,6 +21,7 @@ import * as Constants from '/common/constants.js';
 import * as MetricsData from '/common/metrics-data.js';
 import * as TabsStore from '/common/tabs-store.js';
 import * as TabsUpdate from '/common/tabs-update.js';
+import * as UniqueId from '/common/unique-id.js';
 import * as UserOperationBlocker from '/common/user-operation-blocker.js';
 
 import Tab from '/common/Tab.js';
@@ -435,10 +436,10 @@ async function fixupTabsRestoredFromCache(tabElements, tabs, options = {}) {
 // updating cache
 // ===================================================================
 
-function updateWindowCache(key, value) {
+async function updateWindowCache(key, value) {
   if (!configs.persistCachedTree &&
       browser.storage.session) {
-    const storagKey = `sidebarCache-window${mTargetWindow}-${key}`;
+    const storagKey = `sidebarCache-${await UniqueId.ensureWindowId(mTargetWindow)}-${key}`;
     if (value) {
       const data = {};
       data[storagKey] = value;
@@ -493,7 +494,7 @@ export function markWindowCacheDirty(key) {
 
 async function getWindowCache(key) {
   if (!configs.persistCachedTree) {
-    const storageKey = `sidebarCache-window${mTargetWindow}-${key}`;
+    const storageKey = `sidebarCache-${await UniqueId.ensureWindowId(mTargetWindow)}-${key}`;
     const defaultData = {};
     defaultData[storageKey] = undefined;
     return browser.storage.session.get(defaultData).then(data => {