You are right, this method doesnt work on FF...
The work around I found so far is add id="auto_login" into the checkbox
and then use "cross-browser ID read" script.
So, the final code is:
<script language="javascript">
function get_id(id)
{
if (document.getElementById) return document.getElementById(id);
if (document.all && !document.getElementById) return document.all[id];
if (document.layers) return document.layers[id];
}
</script>
<input type="checkbox" name="auto_login" id="auto_login" value="1" checked/> <span class="smalltext" onClick="alcb=get_id('auto_login');if(alcb.checked) alcb.checked=false; else alcb.checked=true;" style="cursor:default;">{lang_auto_login}</span>
(if you already have get_id() JS function, then you should not use mine.)