--- a/dom/media/MediaResource.cpp
+++ b/dom/media/MediaResource.cpp
@@ -217,17 +217,19 @@ ChannelMediaResource::OnStartRequest(nsI
     hc->GetResponseHeader(NS_LITERAL_CSTRING("Accept-Ranges"),
                           ranges);
     bool acceptsRanges = ranges.EqualsLiteral("bytes");
     // True if this channel will not return an unbounded amount of data
     bool dataIsBounded = false;
 
     int64_t contentLength = -1;
     hc->GetContentLength(&contentLength);
-    if (contentLength >= 0 && responseStatus == HTTP_OK_CODE) {
+    if (contentLength >= 0 &&
+        (responseStatus == HTTP_OK_CODE ||
+         responseStatus == HTTP_PARTIAL_RESPONSE_CODE)) {
       // "OK" status means Content-Length is for the whole resource.
       // Since that's bounded, we know we have a finite-length resource.
       dataIsBounded = true;
     }
 
     // Assume Range requests have a bounded upper limit unless the
     // Content-Range header tells us otherwise.
     bool boundedSeekLimit = true;