--- a/modules/libjar/nsJARChannel.cpp
+++ b/modules/libjar/nsJARChannel.cpp
@@ -948,18 +948,18 @@ nsJARChannel::OnDownloadComplete(MemoryD
         channel->GetSecurityInfo(getter_AddRefs(mSecurityInfo));
 
         nsCOMPtr<nsIHttpChannel> httpChannel(do_QueryInterface(channel));
         if (httpChannel) {
             // We only want to run scripts if the server really intended to
             // send us a JAR file.  Check the server-supplied content type for
             // a JAR type.
             nsAutoCString header;
-            httpChannel->GetResponseHeader(NS_LITERAL_CSTRING("Content-Type"),
-                                           header);
+            Unused << httpChannel->GetResponseHeader(
+              NS_LITERAL_CSTRING("Content-Type"), header);
             nsAutoCString contentType;
             nsAutoCString charset;
             NS_ParseResponseContentType(header, contentType, charset);
             nsAutoCString channelContentType;
             channel->GetContentType(channelContentType);
             mIsUnsafe = !(contentType.Equals(channelContentType) &&
                           (contentType.EqualsLiteral("application/java-archive") ||
                            contentType.EqualsLiteral("application/x-jar")));