Index: lib/filterStorage.js
===================================================================
--- a/lib/filterStorage.js
+++ b/lib/filterStorage.js
@@ -205,36 +205,38 @@
    *   particular group that the filter should be added to
    * @param {number} [position]
    *   position within the subscription at which the filter should be added
    */
   addFilter(filter, subscription, position)
   {
     if (!subscription)
     {
-      if (filter.subscriptions.some(s => s instanceof SpecialSubscription &&
-                                         !s.disabled))
+      for (let currentSubscription of filter.subscriptions)
       {
-        return;   // No need to add
+        if (currentSubscription instanceof SpecialSubscription &&
+            !currentSubscription.disabled)
+        {
+          return;   // No need to add
+        }
       }
       subscription = FilterStorage.getGroupForFilter(filter);
     }
     if (!subscription)
     {
       // No group for this filter exists, create one
       subscription = SpecialSubscription.createForFilter(filter);
       this.addSubscription(subscription);
       return;
     }
 
     if (typeof position == "undefined")
       position = subscription.filters.length;
 
-    if (filter.subscriptions.indexOf(subscription) < 0)
-      filter.subscriptions.push(subscription);
+    filter.subscriptions.add(subscription);
     subscription.filters.splice(position, 0, filter);
     FilterNotifier.triggerListeners("filter.added", filter, subscription,
                                     position);
   },
 
   /**
    * Removes a user-defined filter from the list
    * @param {Filter} filter
@@ -242,21 +244,20 @@
    *      the filter should be removed from (if ommited will be removed from all
    *      subscriptions)
    * @param {number} [position]  position inside the filter group at which the
    *      filter should be removed (if ommited all instances will be removed)
    */
   removeFilter(filter, subscription, position)
   {
     let subscriptions = (
-      subscription ? [subscription] : filter.subscriptions.slice()
+      subscription ? [subscription] : filter.subscriptions
     );
-    for (let i = 0; i < subscriptions.length; i++)
+    for (let currentSubscription of subscriptions)
     {
-      let currentSubscription = subscriptions[i];
       if (currentSubscription instanceof SpecialSubscription)
       {
         let positions = [];
         if (typeof position == "undefined")
         {
           let index = -1;
           do
           {
@@ -270,21 +271,17 @@
 
         for (let j = positions.length - 1; j >= 0; j--)
         {
           let currentPosition = positions[j];
           if (currentSubscription.filters[currentPosition] == filter)
           {
             currentSubscription.filters.splice(currentPosition, 1);
             if (currentSubscription.filters.indexOf(filter) < 0)
-            {
-              let index = filter.subscriptions.indexOf(currentSubscription);
-              if (index >= 0)
-                filter.subscriptions.splice(index, 1);
-            }
+              filter.subscriptions.delete(currentSubscription);
             FilterNotifier.triggerListeners(
               "filter.removed", filter, currentSubscription, currentPosition
             );
           }
         }
       }
     }
   },
@@ -657,35 +654,31 @@
  *   filter subscription that should be connected to its filters
  */
 function addSubscriptionFilters(subscription)
 {
   if (!FilterStorage.knownSubscriptions.has(subscription.url))
     return;
 
   for (let filter of subscription.filters)
-    filter.subscriptions.push(subscription);
+    filter.subscriptions.add(subscription);
 }
 
 /**
  * Removes subscription's filters from the subscription without any
  * notifications.
  * @param {Subscription} subscription filter subscription to be removed
  */
 function removeSubscriptionFilters(subscription)
 {
   if (!FilterStorage.knownSubscriptions.has(subscription.url))
     return;
 
   for (let filter of subscription.filters)
-  {
-    let i = filter.subscriptions.indexOf(subscription);
-    if (i >= 0)
-      filter.subscriptions.splice(i, 1);
-  }
+    filter.subscriptions.delete(subscription);
 }
 
 /**
  * Listener returned by FilterStorage.importData(), parses filter data.
  * @constructor
  */
 function INIParser()
 {
@@ -738,17 +731,17 @@
               {
                 let subscription = this.subscriptions[
                   this.subscriptions.length - 1
                 ];
                 for (let text of this.curObj)
                 {
                   let filter = Filter.fromText(text);
                   subscription.filters.push(filter);
-                  filter.subscriptions.push(subscription);
+                  filter.subscriptions.add(subscription);
                 }
               }
               break;
           }
         }
 
         if (val === null)
           return;