When you click on billgroup and want to see the bill-preview then the preview extension is 'tmp.pdf' so that you can't open the pdf-file.

In accouting.cgi on line 102 the '' has to remove.
Then is file extension valid.

Diff from /srv/web/ipfire/cgi-bin/accounting.cgi

--- /srv/web/ipfire/cgi-bin/accounting.cgi      2014-09-23 18:30:11.983180155 +0200
+++ /root/accounting.cgi        2014-09-23 18:43:40.929825741 +0200
@@ -99,7 +99,7 @@
        print "Content-Type:application/pdf\n";
        my @fileinfo = stat($tempfile);
        print "Content-Length:$fileinfo[7]\n";
-       print "Content-Disposition:attachment;filename=tmp.pdf;\n\n";
+       print "Content-Disposition:attachment;filename='tmp.pdf';\n\n";
        print @fileholder;
        unlink ($tempfile);
        exit (0);