help..window.open working on ie and not in ff

[shotokan99]

i have this function:

function doit(){
if (dow.cpmake.selectedIndex==1 && dow.cpmodel.selectedIndex==0){
window.open('http://www.mysite.com/mycab.cab');
}
}

using ie this using will download the file..however in ff, its not
working. how to do it in ff?
tnx