document.getElementById('btnid').disabled is not working in firefox and chrome

I'm using JavaScript for disabling a button. Works fine in IE but not in FireFox and chrome, here is the script what I'm working on:

function disbtn(e) { 
    if ( someCondition == true ) {
       document.getElementById('btn1').disabled = true;
    } else {
       document.getElementById('btn1').disabled = false;
    }

And in my html I have:

<input type="button" id="btn1" value="submit" />