diff --git a/browser/components/feeds/src/nsFeedSniffer.cpp b/browser/components/feeds/src/nsFeedSniffer.cpp
index 2c5602553586..5ee58b07d875 100644
--- a/browser/components/feeds/src/nsFeedSniffer.cpp
+++ b/browser/components/feeds/src/nsFeedSniffer.cpp
@@ -69,7 +69,7 @@
 #define NS_RDF "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
 #define NS_RSS "http://purl.org/rss/1.0/"
 
-#define MAX_BYTES 512
+#define MAX_BYTES 512u
 
 NS_IMPL_ISUPPORTS3(nsFeedSniffer,
                    nsIContentSniffer,
@@ -324,8 +324,9 @@ nsFeedSniffer::GetMIMETypeFromContent(nsIRequest* request,
   // We cap the number of bytes to scan at MAX_BYTES to prevent picking up 
   // false positives by accidentally reading document content, e.g. a "how to
   // make a feed" page.
-  if (length > MAX_BYTES)
-    length = MAX_BYTES;
+  if (!mDecodedData.IsEmpty()) {
+    length = NS_MIN(mDecodedData.Length(), MAX_BYTES);
+  }
 
   // Thus begins the actual sniffing.
   nsDependentCSubstring dataString((const char*)testData, length);