LoadVars send firefox popup block

function PlayEmbededVideo()
{
    var lvSend:LoadVars = new LoadVars();
    lvSend.strEmbed = "some text";
    lvSend.send("playembed.php", "_blank", "POST");
}

// first case: when i press the button, firefox blocks the new window
myBtn.onPress = function()
{
  PlayEmbededVideo();
}

// -------- onPress and onRelease are not used in the same time, i remove one of them when testing

// second case: when i release the button, firefox displays the new window
myBtn.onRelease = function()
{
  PlayEmbededVideo();
}