Why does the following code call twice the paint bean on FF (when the page is loaded) and never on MSIE 8 ???

On FF, I see the paint object called twice (and the same PDF is always displayed).

<rich:panel>
 <h:panelGrid id="previsu">
 <a4j:mediaOutput element="object" value="constante"
 uriAttribute="data" createContent="#{ReportBean.paint}"
 mimeType="application/pdf" id="pdf-preview" cacheable="false"
 session="true" width="300" height="300">
 </a4j:mediaOutput>
 </h:panelGrid>
 </rich:panel>

Bean


 public void paint(OutputStream out, Object data) throws IOException
 {
 i++;

 File file= null;

 log.info(" Debug called " + i);

// flip flop between two test files...

 if (i % 2 == 0)
 file = new File("c:/temp/File2.pdf");
 else
 file = new File("c:/tempFile1.pdf");

 FileInputStream in=null;
 byte[] bytes;


 try {
 in = new FileInputStream (file);
 long length = file.length();

 if (length > Integer.MAX_VALUE) {
 throw new IOException("The file is too big");
 }

 // Create the byte array to hold the data
 bytes = new byte[(int)length];

 // Read in the bytes
 int offset = 0;
 int numRead = 0;
 while (offset < bytes.length
 && (numRead=in.read(bytes, offset, bytes.length-offset)) >= 0) {
 offset += numRead;
 }

 // Ensure all the bytes have been read in
 if (offset < bytes.length) {
 throw new IOException("The file was not completely read: " + file.getName());
 }

 } finally {
 if (in != null) {
 in.close();
 }
 }

 log.info("Paint " + bytes.length + "bytes");
 out.write(bytes);
 }