Doesn't work for me either, but that isn't surprising considering that Firefox is forced into the Quirks Mode and with the errors on that page.There's at least 4 errors related to JavaScript.
http://validator.w3.org/check?uri=http://www.dewittlumber.com/buildestimator.htm

The question is possibly rather too technical for this forum, although you may get lucky and find an answer.

I assume this is your own website.

If it is a simple problem maybe someone will give you a quick reply. Otherwise I suggest you try elsewhere such as mozillazine http://forums.mozillazine.org/viewforum.php?f=25

(Please post back if you find a solution)

The site tries to refer to the id via var bt=1*document.getElementById("BT").value; , but there is only a name and not an id attribute in the select elements.

This code pasted in the command line in the Web Console (Web Developer > Web Console;Ctrl+Shift+K) should fix this by copying the name attribute to the id attribute.

Paste the code in the command line and press enter.

var s=document.querySelectorAll("select[name]"),i,j=0;for(i=0;S=s[i];i++){if(!S.id){S.id=S.name;j++}}alert(j);