diff --git a/platform/firefox/vapi-webrequest.js b/platform/firefox/vapi-webrequest.js
index c2c1c9043..812c9655b 100644
--- a/platform/firefox/vapi-webrequest.js
+++ b/platform/firefox/vapi-webrequest.js
@@ -30,7 +30,7 @@
     // https://github.com/gorhill/uBlock/issues/2950
     // Firefox 56 does not normalize URLs to ASCII, uBO must do this itself.
     // https://bugzilla.mozilla.org/show_bug.cgi?id=945240
-    let evalMustPunycode = function() {
+    const evalMustPunycode = function() {
         return vAPI.webextFlavor.soup.has('firefox') &&
                vAPI.webextFlavor.major < 57;
     };
@@ -39,16 +39,16 @@
 
     // The real actual webextFlavor value may not be set in stone, so listen
     // for possible future changes.
-    window.addEventListener('webextFlavor', function() {
+    window.addEventListener('webextFlavor', ( ) => {
         mustPunycode = evalMustPunycode();
     }, { once: true });
 
-    let denormalizeTypes = function(aa) {
+    const denormalizeTypes = function(aa) {
         if ( aa.length === 0 ) {
             return Array.from(vAPI.net.validTypes);
         }
-        let out = new Set(),
-            i = aa.length;
+        const out = new Set();
+        let i = aa.length;
         while ( i-- ) {
             let type = aa[i];
             if ( vAPI.net.validTypes.has(type) ) {
@@ -57,13 +57,16 @@
             if ( type === 'image' && vAPI.net.validTypes.has('imageset') ) {
                 out.add('imageset');
             }
+            if ( type === 'sub_frame' ) {
+                out.add('object');
+            }
         }
         return Array.from(out);
     };
 
-    let punycode = self.punycode;
-    let reAsciiHostname  = /^https?:\/\/[0-9a-z_.:@-]+[/?#]/;
-    let parsedURL = new URL('about:blank');
+    const punycode = self.punycode;
+    const reAsciiHostname  = /^https?:\/\/[0-9a-z_.:@-]+[/?#]/;
+    const parsedURL = new URL('about:blank');
 
     vAPI.net.normalizeDetails = function(details) {
         if ( mustPunycode && !reAsciiHostname.test(details.url) ) {
@@ -74,11 +77,11 @@
             );
         }
 
-        let type = details.type;
+        const type = details.type;
 
         // https://github.com/gorhill/uBlock/issues/1493
-        // Chromium 49+/WebExtensions support a new request type: `ping`,
-        // which is fired as a result of using `navigator.sendBeacon`.
+        //   Chromium 49+/WebExtensions support a new request type: `ping`,
+        //   which is fired as a result of using `navigator.sendBeacon`.
         if ( type === 'ping' ) {
             details.type = 'beacon';
             return;
@@ -88,10 +91,24 @@
             details.type = 'image';
             return;
         }
+
+        // https://github.com/uBlockOrigin/uBlock-issues/issues/345
+        //   Re-categorize an embedded object as a `sub_frame` if its
+        //   content type is that of a HTML document.
+        if ( type === 'object' && Array.isArray(details.responseHeaders) ) {
+            for ( const header of details.responseHeaders ) {
+                if ( header.name.toLowerCase() === 'content-type' ) {
+                    if ( header.value.startsWith('text/html') ) {
+                        details.type = 'sub_frame';
+                    }
+                    break;
+                }
+            }
+        }
     };
 
     vAPI.net.denormalizeFilters = function(filters) {
-        let urls = filters.urls || [ '<all_urls>' ];
+        const urls = filters.urls || [ '<all_urls>' ];
         let types = filters.types;
         if ( Array.isArray(types) ) {
             types = denormalizeTypes(types);