If you have a website, hosted on Netfirms, you should have experienced the problem of getting Error 500 even if you have a PHP syntax error in your scripts. I have never seen such a strange solution to make all errors look like 500, but I think this is done because of security requirements.
Anyway, if you need to find out what causes the problem on your hosting accounts, you should visit this page. You can find this page by following CGI and Scripted Language Support link on the main page of your control panel. Hope this post can save some time to you and to support officer at Netfirms, since they don’t have this documented well.