diff --git a/platform/mv3/extension/js/filter-manager.js b/platform/mv3/extension/js/filter-manager.js
index 52839cc64..94a41b7b2 100644
--- a/platform/mv3/extension/js/filter-manager.js
+++ b/platform/mv3/extension/js/filter-manager.js
@@ -38,6 +38,10 @@ import { ubolErr } from './debug.js';
 
 /******************************************************************************/
 
+const perSitePendingIO = new Map();
+
+/******************************************************************************/
+
 export async function selectorsFromCustomFilters(hostname) {
     const promises = [];
     let hn = hostname;
@@ -132,6 +136,9 @@ export async function injectCustomFilters(tabId, frameId, hostname) {
 /******************************************************************************/
 
 export async function registerCustomFilters(context) {
+    if ( perSitePendingIO.size !== 0 ) {
+        await Promise.all(Array.from(perSitePendingIO.values()));
+    }
     const siteKeys = await getAllCustomFilterKeys();
     if ( siteKeys.length === 0 ) { return; }
 
@@ -168,6 +175,20 @@ export async function registerCustomFilters(context) {
 /******************************************************************************/
 
 export async function addCustomFilter(hostname, selector) {
+    const pending = perSitePendingIO.get(hostname);
+    const promise = pending
+        ? pending.then(( ) => addCustomFilterByHostname(hostname, selector))
+        : addCustomFilterByHostname(hostname, selector);
+    perSitePendingIO.set(hostname, promise);
+    promise.then(( ) => {
+        if ( promise !== perSitePendingIO.get(hostname) ) { return; }
+        perSitePendingIO.delete(hostname);
+    });
+    return promise;
+}
+
+async function addCustomFilterByHostname(hostname, selector) {
+    if ( hostname === '' ) { return false; }
     const key = `site.${hostname}`;
     const selectors = await localRead(key) || [];
     if ( selectors.includes(selector) ) { return false; }
@@ -183,9 +204,7 @@ export async function removeCustomFilter(hostname, selector) {
     const promises = [];
     let hn = hostname;
     while ( hn !== '' ) {
-        promises.push(
-            removeCustomFilterByKey(`site.${hn}`, selector).catch(( ) => false)
-        );
+        promises.push(removeCustomFilterByHostname(hn, selector));
         const pos = hn.indexOf('.');
         if ( pos === -1 ) { break; }
         hn = hn.slice(pos + 1);
@@ -194,11 +213,28 @@ export async function removeCustomFilter(hostname, selector) {
     return results.some(a => a);
 }
 
+async function removeCustomFilterByHostname(hostname, selector) {
+    const pending = perSitePendingIO.get(hostname);
+    const key = `site.${hostname}`;
+    const promise = pending
+        ? pending.then(( ) => removeCustomFilterByKey(key, selector))
+        : removeCustomFilterByKey(key, selector);
+    perSitePendingIO.set(hostname, promise);
+    promise.then(( ) => {
+        if ( promise !== perSitePendingIO.get(hostname) ) { return; }
+        perSitePendingIO.delete(hostname);
+    });
+    return promise;
+}
+
 async function removeCustomFilterByKey(key, selector) {
     const selectors = await localRead(key);
     if ( selectors === undefined ) { return false; }
-    const i = selectors.indexOf(selector);
-    if ( i === -1 ) { return false; }
+    let i = selectors.indexOf(selector);
+    if ( i === -1 ) {
+        i = selectors.indexOf(`0${selector}`);
+        if ( i === -1 ) { return false; }
+    }
     selectors.splice(i, 1);
     if ( selectors.length !== 0 ) {
         await localWrite(key, selectors);