diff --git a/webextensions/background/background-cache.js b/webextensions/background/background-cache.js
index cc2882124..279b8c5a7 100644
--- a/webextensions/background/background-cache.js
+++ b/webextensions/background/background-cache.js
@@ -517,10 +517,27 @@ Tab.onHidden.addListener(tab => {
   reserveToCacheTree(tab.windowId);
 });
 
+browser.runtime.onMessage.addListener((message, _sender) => {
+  switch (message && message.type) {
+    case Constants.kCOMMAND_GET_ON_MEMORY_CACHE:
+      return mCaches[message.key];
+
+    case Constants.kCOMMAND_SET_ON_MEMORY_CACHE:
+      if (message.value)
+        mCaches[message.key] = message.value;
+      else
+        delete mCaches[message.key];
+      return;
+
+    default:
+      return;
+  }
+});
+
 browser.windows.onRemoved.addListener(async windowId => {
-  const storageKeyPrefix = `backgroundCache-${await UniqueId.ensureWindowId(windowId)}-`;
+  const storageKeyPart = `Cache-${await UniqueId.ensureWindowId(windowId)}-`;
   for (const key in mCaches) {
-    if (key.startsWith(storageKeyPrefix))
+    if (key.includes(storageKeyPart))
       delete mCaches[key];
   }
 });
diff --git a/webextensions/common/constants.js b/webextensions/common/constants.js
index 4f2ef1de6..9ca9647d4 100644
--- a/webextensions/common/constants.js
+++ b/webextensions/common/constants.js
@@ -92,6 +92,9 @@ export const kCOMMAND_UNBLOCK_USER_OPERATIONS = 'treestyletab:unblock-user-opera
 export const kCOMMAND_PROGRESS_USER_OPERATIONS = 'treestyletab:progress-user-operations';
 export const kCOMMAND_BROADCAST_TAB_STATE     = 'treestyletab:broadcast-tab-state';
 
+export const kCOMMAND_SET_ON_MEMORY_CACHE     = 'treestyletab:set-on-memory-cache';
+export const kCOMMAND_GET_ON_MEMORY_CACHE     = 'treestyletab:get-on-memory-cache';
+
 export const kCOMMAND_BOOKMARK_TAB_WITH_DIALOG  = 'treestyletab:bookmark-tab-with-dialog';
 export const kCOMMAND_BOOKMARK_TABS_WITH_DIALOG = 'treestyletab:bookmark-tabs-with-dialog';
 
diff --git a/webextensions/sidebar/sidebar-cache.js b/webextensions/sidebar/sidebar-cache.js
index f5ab172a4..53cea5488 100644
--- a/webextensions/sidebar/sidebar-cache.js
+++ b/webextensions/sidebar/sidebar-cache.js
@@ -437,17 +437,13 @@ async function fixupTabsRestoredFromCache(tabElements, tabs, options = {}) {
 // ===================================================================
 
 async function updateWindowCache(key, value) {
-  if (!configs.persistCachedTree &&
-      browser.storage.session) {
+  if (!configs.persistCachedTree) {
     const storagKey = `sidebarCache-${await UniqueId.ensureWindowId(mTargetWindow)}-${key}`;
-    if (value) {
-      const data = {};
-      data[storagKey] = value;
-      browser.storage.session.set(data);
-    }
-    else {
-      browser.storage.session.remove(storagKey);
-    }
+    browser.runtime.sendMessage({
+      type: Constants.kCOMMAND_SET_ON_MEMORY_CACHE,
+      key:  storagKey,
+      value,
+    });
     return;
   }
 
@@ -495,10 +491,9 @@ export function markWindowCacheDirty(key) {
 async function getWindowCache(key) {
   if (!configs.persistCachedTree) {
     const storageKey = `sidebarCache-${await UniqueId.ensureWindowId(mTargetWindow)}-${key}`;
-    const defaultData = {};
-    defaultData[storageKey] = undefined;
-    return browser.storage.session.get(defaultData).then(data => {
-      return data[storageKey];
+    return browser.runtime.sendMessage({
+      type: Constants.kCOMMAND_GET_ON_MEMORY_CACHE,
+      key:  storageKey,
     });
   }