diff --git a/platform/firefox/frameModule.js b/platform/firefox/frameModule.js
index ef5f04c2b..6f2c284c2 100644
--- a/platform/firefox/frameModule.js
+++ b/platform/firefox/frameModule.js
@@ -207,11 +207,7 @@ var contentObserver = {
         // - Enable uBlock
         // - Services and all other global variables are undefined
         // Hopefully will eventually understand why this happens.
-        if ( Services === undefined ) {
-            return this.ACCEPT;
-        }
-
-        if ( !context ) {
+        if ( Services === undefined || !context ) {
             return this.ACCEPT;
         }
 
@@ -219,6 +215,13 @@ var contentObserver = {
             this.handlePopup(location, origin, context);
         }
 
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1232354
+        // For modern versions of Firefox, the frameId/parentFrameId
+        // information can be found in channel.loadInfo of the HTTP observer.
+        if ( this.canE10S ) {
+            return this.ACCEPT;
+        }
+
         if ( !location.schemeIs('http') && !location.schemeIs('https') ) {
             return this.ACCEPT;
         }
@@ -245,19 +248,12 @@ var contentObserver = {
             return this.ACCEPT;
         }
 
-        // https://bugzilla.mozilla.org/show_bug.cgi?id=1232354
-        // For top-level resources, no need to send information to the
-        // main process.
-        let isTopContext = context === context.top;
-        if ( isTopContext && this.canE10S ) {
-            return this.ACCEPT;
-        }
-
         let messageManager = getMessageManager(context);
         if ( messageManager === null ) {
             return this.ACCEPT;
         }
 
+        let isTopContext = context === context.top;
         var parentFrameId;
         if ( isTopContext ) {
             parentFrameId = -1;
diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index 52485c6fd..b5df649b1 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -2026,11 +2026,11 @@ var httpObserver = {
         if ( bucket === undefined ) {
             return null;
         }
-        var i = bucket.charCodeAt(0);
+        var i = bucket.charCodeAt(bucket.length - 1);
         if ( bucket.length === 1 ) {
             this.pendingURLToIndex.delete(url);
         } else {
-            this.pendingURLToIndex.set(url, bucket.slice(1));
+            this.pendingURLToIndex.set(url, bucket.slice(0, -1));
         }
         var preq = this.pendingRingBuffer[i];
         preq._key = ''; // mark as "serviced"
@@ -2225,8 +2225,15 @@ var httpObserver = {
         var pendingRequest = this.lookupPendingRequest(URI.spec);
 
         // https://github.com/gorhill/uMatrix/issues/390#issuecomment-155759004
-        var rawtype = 1;
-        var loadInfo = channel.loadInfo;
+        var loadInfo = channel.loadInfo,
+            rawtype = 1,
+            frameId = 0,
+            parentFrameId = -1;
+
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1232354
+        // https://dxr.mozilla.org/mozilla-central/source/toolkit/modules/addons/WebRequest.jsm#537-553
+        // For modern Firefox, loadInfo contains the information about the
+        // context of the network request.
         if ( loadInfo ) {
             rawtype = loadInfo.externalContentPolicyType !== undefined ?
                 loadInfo.externalContentPolicyType :
@@ -2234,6 +2241,11 @@ var httpObserver = {
             if ( !rawtype ) {
                 rawtype = 1;
             }
+            frameId = loadInfo.frameOuterWindowID ? loadInfo.frameOuterWindowID : loadInfo.outerWindowID;
+            parentFrameId = loadInfo.frameOuterWindowID ? loadInfo.outerWindowID : loadInfo.parentOuterWindowID;
+            if ( frameId === parentFrameId ) {
+                parentFrameId = -1;
+            }
         }
 
         if ( pendingRequest !== null ) {
@@ -2259,6 +2271,8 @@ var httpObserver = {
             pendingRequest = this.syntheticPendingRequest;
             pendingRequest.tabId = this.tabIdFromChannel(channel);
             pendingRequest.rawtype = rawtype;
+            pendingRequest.frameId = frameId;
+            pendingRequest.parentFrameId = parentFrameId;
         }
 
         if ( this.handleRequest(channel, URI, pendingRequest) ) {
diff --git a/src/js/traffic.js b/src/js/traffic.js
index b57e3aa75..4d6fb1f0a 100644
--- a/src/js/traffic.js
+++ b/src/js/traffic.js
@@ -77,10 +77,9 @@ var onBeforeRequest = function(details) {
     // > the outer frame.
     // > (ref: https://developer.chrome.com/extensions/webRequest)
     var isFrame = requestType === 'sub_frame';
-    var frameId = isFrame ? details.parentFrameId : details.frameId;
 
     // https://github.com/chrisaljoudi/uBlock/issues/114
-    var requestContext = pageStore.createContextFromFrameId(frameId);
+    var requestContext = pageStore.createContextFromFrameId(isFrame ? details.parentFrameId : details.frameId);
 
     // Setup context and evaluate
     var requestURL = details.url;
@@ -109,9 +108,8 @@ var onBeforeRequest = function(details) {
     // Not blocked
     if ( µb.isAllowResult(result) ) {
         // https://github.com/chrisaljoudi/uBlock/issues/114
-        frameId = details.frameId;
-        if ( frameId > 0 && isFrame ) {
-            pageStore.setFrame(frameId, requestURL);
+        if ( details.parentFrameId !== -1 && isFrame ) {
+            pageStore.setFrame(details.frameId, requestURL);
         }
         requestContext.dispose();
         return;