[Deff]

<style>
#a,#b{
 margin: 0;
 padding: 2px 12px;
 height:20px;
 width:530px;
}

</style>
<div id=a style="border:blue 1px solid;">
<div id=blockfirm1>blockfirm1</div>
</div>

<br/>
<div id=b style="border:red 1px solid;">
<div id=blockfirm2  style="display:none;">blockfirm2</div>
</div>

<br/>

<button onclick="
var a=document.getElementById('blockfirm1');
var b=document.getElementById('blockfirm2');
if(a.style.display=='block'||a.style.display==''){
 a.style.display='none';b.style.display='block';
} else {if (b.style.display=='block'||b.style.display==''){
 b.style.display='none';a.style.display='block';
}}">Change</button>