--- a/dom/media/test/midflight-redirect.sjs
+++ b/dom/media/test/midflight-redirect.sjs
@@ -43,26 +43,31 @@ function handleRequest(request, response
     response.setStatusLine(request.httpVersion, 303, "See Other");
     let url = "http://" + origin +
               "/tests/dom/media/test/midflight-redirect.sjs?redirected&" + query;
     response.setHeader("Location", url);
     response.setHeader("Content-Type", "text/html");
     return;
   }
 
-  if (from == 0 && !redirected) {
-    to = Math.min(bytes.length / 4, 200);
-  } else {
-    to = to || Math.max(from, bytes.length - 1);
+  if (isNaN(to)) {
+    to = bytes.length - 1;
   }
 
+  if (from == 0 && !redirected) {
+    to = parseInt(parseQuery(query, "redirectAt")) || Math.floor(bytes.length / 4);
+  }
+  to = Math.min(to, bytes.length - 1);
+
+  // Note: 'to' is the first index *excluded*, so we need (to + 1)
+  // in the substring end here.
   byterange = bytes.substring(from, to + 1);
 
-  let contentRange = "bytes "+ from +"-"+ to +"/"+ bytes.length;
-  let contentLength = (to - from + 1).toString();
+  let contentRange = "bytes " + from + "-" + to + "/" + bytes.length;
+  let contentLength = byterange.length.toString();
 
   response.setStatusLine(request.httpVersion, 206, "Partial Content");
   response.setHeader("Content-Range", contentRange);
   response.setHeader("Content-Length", contentLength, false);
   response.setHeader("Content-Type", type, false);
   response.setHeader("Accept-Ranges", "bytes", false);
   response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");
   if (redirected && useCors) {

--- a/dom/media/test/test_midflight_redirect_blocked.html
+++ b/dom/media/test/test_midflight_redirect_blocked.html
@@ -37,20 +37,23 @@
           }, false);
 
           element.addEventListener("error", ()=>{
             resolve(false);
             removeNodeAndSource(element);
           }, false);
 
           var noise = Math.floor(Math.random() * 100000000);
+          // Note: request redirect before the end of metadata, otherwise we won't
+          // error before metadata has loaded if mixed origins are allowed.
           element.src = "midflight-redirect.sjs?resource=" + test.name
                       + (useCors ? "&cors" : "")
                       + "&type=" + test.type
-                      + "&noise=" + noise;
+                      + "&noise=" + noise
+                      + "&redirectAt=200";
           element.preload = "metadata";
           document.body.appendChild(element);
           element.load()
         });
       }
 
       let v = document.createElement("video");
       const testCases = gSmallTests.filter(t => v.canPlayType(t.type));