diff --git a/modules/libpr0n/src/imgLoader.cpp b/modules/libpr0n/src/imgLoader.cpp
index 39952dca00c..bac1880bcca 100644
--- a/modules/libpr0n/src/imgLoader.cpp
+++ b/modules/libpr0n/src/imgLoader.cpp
@@ -1459,15 +1459,21 @@ NS_IMETHODIMP imgLoader::GetMIMETypeFromContent(nsIRequest* aRequest,
 nsresult imgLoader::GetMimeTypeFromContent(const char* aContents, PRUint32 aLength, nsACString& aContentType)
 {
   /* Is it a GIF? */
-  if (aLength >= 4 && !nsCRT::strncmp(aContents, "GIF8", 4))  {
+  if (aLength >= 6 && (!nsCRT::strncmp(aContents, "GIF87a", 6) ||
+                       !nsCRT::strncmp(aContents, "GIF89a", 6)))
+  {
     aContentType.AssignLiteral("image/gif");
   }
 
   /* or a PNG? */
-  else if (aLength >= 4 && ((unsigned char)aContents[0]==0x89 &&
+  else if (aLength >= 8 && ((unsigned char)aContents[0]==0x89 &&
                    (unsigned char)aContents[1]==0x50 &&
                    (unsigned char)aContents[2]==0x4E &&
-                   (unsigned char)aContents[3]==0x47))
+                   (unsigned char)aContents[3]==0x47 &&
+                   (unsigned char)aContents[4]==0x0D &&
+                   (unsigned char)aContents[5]==0x0A &&
+                   (unsigned char)aContents[6]==0x1A &&
+                   (unsigned char)aContents[7]==0x0A))
   { 
     aContentType.AssignLiteral("image/png");
   }
diff --git a/netwerk/streamconv/converters/nsUnknownDecoder.cpp b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
index 3ceeeb46314..fa989f54318 100644
--- a/netwerk/streamconv/converters/nsUnknownDecoder.cpp
+++ b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
@@ -60,7 +60,7 @@
 #include "nsNetCID.h"
 
 
-#define MAX_BUFFER_SIZE 1024
+#define MAX_BUFFER_SIZE 512
 
 nsUnknownDecoder::nsUnknownDecoder()
   : mBuffer(nsnull)
@@ -313,7 +313,6 @@ nsUnknownDecoder::nsSnifferEntry nsUnknownDecoder::sSnifferEntries[] = {
   SNIFFER_ENTRY("%PDF-", APPLICATION_PDF),
 
   SNIFFER_ENTRY("%!PS-Adobe-", APPLICATION_POSTSCRIPT),
-  SNIFFER_ENTRY("%! PS-Adobe-", APPLICATION_POSTSCRIPT),
 
   // Files that start with mailbox delimiters let's provisionally call
   // text/plain