Exclude cached resources from large-media-element blocking

Related feedback:
- gorhill#1390 (comment)

src/js/traffic.js

@@ -451,17 +451,17 @@ const onHeadersReceived = function(details) {
     }
     if ( pageStore.getNetFilteringSwitch(fctxt) === false ) { return; }
 
-    // Keep in mind response headers will be modified in-place if needed, so
-    // `details.responseHeaders` will always point to the modified response
-    // headers.
-    const responseHeaders = details.responseHeaders;
-
     if ( requestType === 'image' || requestType === 'media' ) {
-        return foilLargeMediaElement(fctxt, pageStore, responseHeaders);
+        return foilLargeMediaElement(details, fctxt, pageStore);
     }
 
     if ( isDoc === false ) { return; }
 
+    // Keep in mind response headers will be modified in-place if needed, so
+    // `details.responseHeaders` will always point to the modified response
+    // headers.
+    const responseHeaders = details.responseHeaders;
+
     // https://github.com/gorhill/uBlock/issues/2813
     //   Disable the blocking of large media elements if the document is itself
     //   a media element: the resource was not prevented from loading so no
@@ -939,14 +939,21 @@ const injectCSP = function(fctxt, pageStore, responseHeaders) {
 /******************************************************************************/
 
 // https://github.com/gorhill/uBlock/issues/1163
-//   "Block elements by size"
+//   "Block elements by size".
+// https://github.com/gorhill/uBlock/issues/1390#issuecomment-187310719
+//   Do not foil when the media element is fetched from the browser
+//   cache. This works only when the webext API supports the `fromCache`
+//   property (Firefox).
+
+const foilLargeMediaElement = function(details, fctxt, pageStore) {
+    if ( details.fromCache === true ) { return; }
 
-const foilLargeMediaElement = function(fctxt, pageStore, responseHeaders) {
     let size = 0;
     if ( µBlock.userSettings.largeMediaSize !== 0 ) {
-        const i = headerIndexFromName('content-length', responseHeaders);
+        const headers = details.responseHeaders;
+        const i = headerIndexFromName('content-length', headers);
         if ( i === -1 ) { return; }
-        size = parseInt(responseHeaders[i].value, 10) || 0;
+        size = parseInt(headers[i].value, 10) || 0;
     }
 
     const result = pageStore.filterLargeMediaElement(fctxt, size);