Definitely something wrong with the server as the '500' error is a "server encountered an unexpected condition which prevented it from fulfilling the request" error.

If it was something wrong with your client, you should have gotten a '4xx' type of error.

Check the headers which Firefox sends and receives (use the extension "Live HTTP headers") and compare that with the headers your script sends and receives and see what the difference is. It may be that the server disallows 'robot'-type of access, but the response should then be a '403' error ("forbidden").