Not sure if anyone else is having the same problem but when I come to download any builds whether it's general or modified the download doesn't complete, this is the same for IE as well as firefox. Is this an issue with your server timing out or traffic on the WAN? I've spen nearly a day re-downloading a 183 Mb file but it has never completed successfully.
You might benefit from a firefox extension named 'down them all' -- which, if you have the local bandwidth, will greatly shorten the time of a download.
But if you already have the standard slax distro, then you don't need to download 'builds' -- just get the modules you want, since you already have the basic slax part, no need to download them again.
You might also find the same slax modules at various places on the web -- try putting the exact name of a slax module into a google search box, chances are people near you already have it on an ftp page.