firefox ignore javascript httprequest setRequestHeader ("Content-Encoding", "gzip")

 function loadModel(JsonUrl,m) {
   var request = new XMLHttpRequest();
   request.open("GET", JsonUrl);
   request.setRequestHeader("Content-Encoding", "gzip");
   request.setRequestHeader("Content-Type", "text/plain");
   request.onreadystatechange = function() {
     if (request.readyState == 4) {

     	      	ModelArray[m][5]=JSON.parse(request.responseText);
     	      	FindMinMax(ModelArray[m][5].vertexPositions);
	      	  

I am calling JSON files from my Javascript httprequest to represent vertexcoordinates and so on for WebGL. Its work nice with pure json but Firefox ignore *.json.gz files what I requare by setting javascript code: function loadModel(JsonUrl,m) { var request = new XMLHttpRequest(); request.open("GET", JsonUrl); request.setRequestHeader("Content-Encoding", "gzip"); request.setRequestHeader("Content-Type", "text/plain"); request.onreadystatechange = function() { if (request.readyState == 4) { ModelArray[m][5]=JSON.parse(request.responseText); FindMinMax(ModelArray[m][5].vertexPositions); if((m+1)==ModelArray.length && loadSeparate!="true"){ for(m=0;m