diff --git a/src/client/account.ts b/src/client/account.ts
index c539673c0d..48fe76146c 100644
--- a/src/client/account.ts
+++ b/src/client/account.ts
@@ -1,4 +1,4 @@
-import { get, set } from 'idb-keyval';
+import { get, set } from '@/scripts/idb-proxy';
 import { reactive } from 'vue';
 import { apiUrl } from '@/config';
 import { waiting } from '@/os';
diff --git a/src/client/components/sidebar.vue b/src/client/components/sidebar.vue
index 8eace36f19..476c7f8734 100644
--- a/src/client/components/sidebar.vue
+++ b/src/client/components/sidebar.vue
@@ -242,7 +242,7 @@ export default defineComponent({
 		addAcount() {
 			os.popup(import('./signin-dialog.vue'), {}, {
 				done: async res => {
-					addAccount(res.id, res.i);
+					await addAccount(res.id, res.i);
 					os.success();
 				},
 			}, 'closed');
diff --git a/src/client/init.ts b/src/client/init.ts
index 7385c59de2..21bdbee5a8 100644
--- a/src/client/init.ts
+++ b/src/client/init.ts
@@ -4,7 +4,7 @@
 
 import '@/style.scss';
 
-import { set } from 'idb-keyval';
+import { set } from '@/scripts/idb-proxy';
 
 // TODO: そのうち消す
 if (localStorage.getItem('vuex') != null) {
@@ -69,9 +69,9 @@ import { isMobile } from '@/scripts/is-mobile';
 import { getThemes } from '@/theme-store';
 import { initializeSw } from '@/scripts/initialize-sw';
 import { reload, reloadChannel } from '@/scripts/unison-reload';
+import { reactionPicker } from '@/scripts/reaction-picker';
 import { deleteLoginId } from '@/scripts/login-id';
 import { getAccountFromId } from '@/scripts/get-account-from-id';
-import { reactionPicker } from '@/scripts/reaction-picker';
 
 console.info(`Misskey v${version}`);
 
diff --git a/src/client/scripts/get-account-from-id.ts b/src/client/scripts/get-account-from-id.ts
index be4cfaeba4..ba3adceecc 100644
--- a/src/client/scripts/get-account-from-id.ts
+++ b/src/client/scripts/get-account-from-id.ts
@@ -1,4 +1,4 @@
-import { get } from 'idb-keyval';
+import { get } from '@/scripts/idb-proxy';
 
 export async function getAccountFromId(id: string) {
 	const accounts = await get('accounts') as { token: string; id: string; }[];
diff --git a/src/client/scripts/idb-proxy.ts b/src/client/scripts/idb-proxy.ts
new file mode 100644
index 0000000000..c0d01e6292
--- /dev/null
+++ b/src/client/scripts/idb-proxy.ts
@@ -0,0 +1,34 @@
+// FirefoxのプライベートモードなどではindexedDBが使用不可能なので、使う
+import {
+	get as iget,
+	set as iset,
+	del as idel,
+} from 'idb-keyval';
+
+const fallbackName = (key: string) => `idbfallback::${key}`;
+
+let idbAvailable = typeof window !== 'undefined' ? !!window.indexedDB : true;
+
+if (idbAvailable) {
+	try {
+		const request = indexedDB.open('keyval-store');
+		if (request.error) idbAvailable = false;
+	} catch (e) {
+		idbAvailable = false;
+	}
+}
+
+export async function get(key: string) {
+	if (idbAvailable) return iget(key);
+	return JSON.parse(localStorage.getItem(fallbackName(key)));
+}
+
+export async function set(key: string, val: any) {
+	if (idbAvailable) return iset(key, val);
+	return localStorage.setItem(fallbackName(key), JSON.stringify(val));
+}
+
+export async function del(key: string) {
+	if (idbAvailable) return idel(key);
+	return localStorage.removeItem(fallbackName(key));
+}
diff --git a/src/client/sw/sw.ts b/src/client/sw/sw.ts
index 5b17855e08..6f78f3cb38 100644
--- a/src/client/sw/sw.ts
+++ b/src/client/sw/sw.ts
@@ -32,7 +32,10 @@ self.addEventListener('activate', ev => {
 
 //#region When: Fetching
 self.addEventListener('fetch', ev => {
-	// Nothing to do
+	ev.respondWith(
+		fetch(ev.request)
+		.catch(() => new Response(`Offline. Service Worker @${_VERSION_}`, { status: 200 }))
+	);
 });
 //#endregion
 
diff --git a/src/client/ui/_common_/common.vue b/src/client/ui/_common_/common.vue
index 8709f5a686..26e3a5b35e 100644
--- a/src/client/ui/_common_/common.vue
+++ b/src/client/ui/_common_/common.vue
@@ -55,44 +55,46 @@ export default defineComponent({
 			const sideViewHook = inject('sideViewHook', null);
 
 			//#region Listen message from SW
-			navigator.serviceWorker.addEventListener('message', ev => {
-				if (_DEV_) {
-					console.log('sw msg', ev.data);
-				}
+			if ('serviceWorker' in navigator) {
+				navigator.serviceWorker.addEventListener('message', ev => {
+					if (_DEV_) {
+						console.log('sw msg', ev.data);
+					}
 
-				const data = ev.data as SwMessage;
-				if (data.type !== 'order') return;
+					const data = ev.data as SwMessage;
+					if (data.type !== 'order') return;
 
-				if (data.loginId !== $i?.id) {
-					return getAccountFromId(data.loginId).then(account => {
-						if (!account) return;
-						return login(account.token, data.url);
-					});
-				}
+					if (data.loginId !== $i?.id) {
+						return getAccountFromId(data.loginId).then(account => {
+							if (!account) return;
+							return login(account.token, data.url);
+						});
+					}
 
-				switch (data.order) {
-					case 'post':
-						return post(data.options);
-					case 'push':
-						if (data.url.startsWith('/my/messaging')) {
-							if (router.currentRoute.value.path === data.url) return;
-							if (ColdDeviceStorage.get('chatOpenBehavior') === 'window') return pageWindow(data.url);
-							if (ColdDeviceStorage.get('chatOpenBehavior') === 'popout') return popout(data.url);
-						}
-						if (router.currentRoute.value.path === data.url) {
-							return window.scroll({ top: 0, behavior: 'smooth' });
-						}
-						if (navHook) {
-							return navHook(data.url);
-						}
-						if (sideViewHook && defaultStore.state.defaultSideView && data.url !== '/') {
-							return sideViewHook(data.url);
-						}
-						return router.push(data.url);
-					default:
-						return;
-				}
-			});
+					switch (data.order) {
+						case 'post':
+							return post(data.options);
+						case 'push':
+							if (data.url.startsWith('/my/messaging')) {
+								if (router.currentRoute.value.path === data.url) return;
+								if (ColdDeviceStorage.get('chatOpenBehavior') === 'window') return pageWindow(data.url);
+								if (ColdDeviceStorage.get('chatOpenBehavior') === 'popout') return popout(data.url);
+							}
+							if (router.currentRoute.value.path === data.url) {
+								return window.scroll({ top: 0, behavior: 'smooth' });
+							}
+							if (navHook) {
+								return navHook(data.url);
+							}
+							if (sideViewHook && defaultStore.state.defaultSideView && data.url !== '/') {
+								return sideViewHook(data.url);
+							}
+							return router.push(data.url);
+						default:
+							return;
+					}
+				});
+			}
 		}
 
 		return {