works on Internet Explorer not on Mozilla

I have a function that validates the input onChange().In case of an error the function gives an alert message and keeps the focus on the same text box.I achieve this by the following snippet of code in the function using
if (alertbox!="") {alert(alertbox); entered.select(); entered.focus();
This worlks on Internet Explorer but does not on mozilla...

Please Help me on this....

Thanks

In case I am not clear I am attaching the code and the function call

Fuunction Call:
onChange="return numberValidation(this,1,10,'The Number Of Elements should be an Integer within the Range ','Integer'

Function

function numberValidation(entered, min, max, alertbox, datatype)
{
with (entered)
{
checkvalue=parseFloat(value);
if (datatype)
{smalldatatype=datatype.toLowerCase();
if (smalldatatype.charAt(0)=="i")
{checkvalue=parseInt(value); if (value.indexOf(".")!=-1) {checkvalue=checkvalue+1}};
}
if ((parseFloat(min)==min && value.length<min) || (parseFloat(max)==max && value.length>max) || value!=checkvalue)
{if (alertbox!="") {alert(alertbox); entered.select(); entered.focus();
} return false;}
else {return true;}
}
}

I don't think that onChange is supported by mozilla... Not sure, but I heard that somewhere.

I don't think so. Check if there are errors in Javascript console