Snow Script,I works well both in IE and FireFox

/*下雪snow - Start*/

var no = 10; // number of snows

var speed = 20; // smaller number moves the snows faster

var heart = "images/snow.gif";

var flag = new Array();

var ns4up = (document.layers) ? 1 : 0; // browser sniffer

var ie4up = (document.all) ? 1 : 0;

var dom = (document.getElementById) ? 1 : 0;

// coordinate and position variables

var dx = new Array();

var xp = new Array();

var yp = new Array();

// amplitude and step variables

var amx = new Array();

var amy = new Array();

var stx = new Array();

var sty = new Array();

var i, doc_width = 800, doc_height = 600;

if (ie4up) {

doc_width = document.body.clientWidth;

doc_height = document.body.scrollTop+document.body.clientHeight;

}

else {

doc_width = window.innerWidth;

doc_height = window.pageYOffset+window.innerHeight;

}

for (i = 0; i < no; ++ i) {

dx[i] = 0; // set coordinate variables

xp[i] = Math.random()*(doc_width-30)+10; // set position variables

yp[i] = Math.random()*doc_height;

amy[i] = 12+ Math.random()*20; // set amplitude variables

amx[i] = 10+ Math.random()*40;

stx[i] = 0.02 + Math.random()/10; // set step variables

sty[i] = 0.7 + Math.random(); // set step variables

flag[i] = (Math.random()>0.5)?1:0;

if (ns4up) { // set layers

 document.write("<layer id="dot"+ i +"" name="dot"+ i +"" left="150" top="150" visibility="show"><img src="" + heart+ "" border="0"></layer>");

}

else{

 document.write("<div id="dot"+ i +"" style="POSITION: absolute; Z-INDEX: "+ i +"; VISIBILITY: visible; TOP: 150px; LEFT: 150px;"><img src="" + heart+ "" border="0"></div>");

}

}

function snow(){

for (i = 0; i < no; ++ i) { // iterate for every dot

 if (yp[i] > doc_height-50) {

  xp[i] = 10+ Math.random()*(doc_width-amx[i]-30);

  yp[i] = 0;

  stx[i] = 0.02 + Math.random()/10;

  sty[i] = 0.7 + Math.random();

  flag[i]=(Math.random()<0.5)?1:0;

  if (ie4up) {

   doc_width = document.body.clientWidth;

   doc_height = document.body.clientHeight;

  }

  else{

   doc_width = window.innerWidth;

   doc_height = window.innerHeight;

  }

 }

 if (flag[i])

  dx[i] += stx[i];

 else

  dx[i] -= stx[i];

 if (Math.abs(dx[i]) > Math.PI) {

  yp[i]+=Math.abs(amy[i]*dx[i]);

  xp[i]+=amx[i]*dx[i];

  dx[i]=0;

  flag[i]=!flag[i];

 }

 idot="dot"+i;

 c=(dom)? document.getElementById(idot).style : (ie4up)? document.all[idot].style : (ns4up)? document.layers[idot] : "";

 theTop=yp[i] + amy[i]*(Math.abs(Math.sin(dx[i])+dx[i]));

 theLeft=xp[i] + amx[i]*dx[i];

 if (ns4up) {

  c.top=theTop;

  c.left=theLeft;

 }

 else if(ie4up){

  c.pixelTop=theTop;

  c.pixelLeft=theLeft;

 }

 else if(dom){

  c.top=theTop+"px";

  c.left=theLeft+"px";

 };

}

}

/*下雪 snow - End*/

window.setInterval("snow()", speed);//下雪，snow

<script type="text/javascript" src="snow.js"></script>