--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -7434,16 +7434,17 @@ private:
     int64_t mProgressMax;
 };
 
 NS_IMETHODIMP
 nsHttpChannel::OnDataAvailable(nsIRequest *request, nsISupports *ctxt,
                                nsIInputStream *input,
                                uint64_t offset, uint32_t count)
 {
+    nsresult rv;
     PROFILER_LABEL("nsHttpChannel", "OnDataAvailable",
         js::ProfileEntry::Category::NETWORK);
 
     LOG(("nsHttpChannel::OnDataAvailable [this=%p request=%p offset=%" PRIu64
          " count=%" PRIu32 "]\n",
         this, request, offset, count));
 
     LOG(("  requestFromCache: %d mFirstResponseSource: %d\n",
@@ -7478,17 +7479,21 @@ nsHttpChannel::OnDataAvailable(nsIReques
         else
             transportStatus = NS_NET_STATUS_RECEIVING_FROM;
 
         // mResponseHead may reference new or cached headers, but either way it
         // holds our best estimate of the total content length.  Even in the case
         // of a byte range request, the content length stored in the cached
         // response headers is what we want to use here.
 
-        int64_t progressMax(mResponseHead->ContentLength());
+        int64_t progressMax = -1;
+        rv = GetContentLength(&progressMax);
+        if (NS_FAILED(rv)) {
+            NS_WARNING("GetContentLength failed");
+        }
         int64_t progress = mLogicalOffset + count;
 
         if ((progress > progressMax) && (progressMax != -1)) {
             NS_WARNING("unexpected progress values - "
                        "is server exceeding content length?");
         }
 
         // make sure params are in range for js
@@ -7498,17 +7503,17 @@ nsHttpChannel::OnDataAvailable(nsIReques
 
         if (!InScriptableRange(progress)) {
             progress = -1;
         }
 
         if (NS_IsMainThread()) {
             OnTransportStatus(nullptr, transportStatus, progress, progressMax);
         } else {
-            nsresult rv = NS_DispatchToMainThread(
+            rv = NS_DispatchToMainThread(
                 new OnTransportStatusAsyncEvent(this, transportStatus,
                                                 progress, progressMax));
             NS_ENSURE_SUCCESS(rv, rv);
         }
 
         //
         // we have to manually keep the logical offset of the stream up-to-date.
         // we cannot depend solely on the offset provided, since we may have