If I try your url with LiveHttpHeaders add-on, I get very strange results.
It looks like the server is sending some very bad headers.
I can not read the site, maybe I need to log in to get better results.
http://www.cnc-tekniikka.com/CNC-forum1/index.php?action=dlattach;topic=397.0;attach=656 There is no Content Length information, the Content-type is probably wrong.
In Firefox 8 this Content-Disposition probably does not work too.

GET /CNC-forum1/index.php?action=dlattach;topic=397.0;attach=656 HTTP/1.1
Host: www.cnc-tekniikka.com
User-Agent: Mozilla/5.0 (Windows NT 6.0; rv:8.0) Gecko/20100101 Firefox/8.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: nl,en-us;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive

HTTP/1.1 200 OK
Date: Thu, 08 Dec 2011 10:37:01 GMT
Server: Apache
X-Powered-By: PHP/5.2.17
Vary: Accept-Encoding
Content-Encoding: none
Expires: Fri, 07 Dec 2012 10:37:01 GMT
Accept-Ranges: bytes
Connection: close
Etag: "656Kuva 030.jpg1240148623"
Content-Disposition: attachment; filename*="UTF-8Kuva 030.jpg"
Cache-Control: no-cache
Set-Cookie: PHPSESSID=37353dc254e3d22e7990c8124d140b40; path=/
Last-Modified: Sun, 19 Apr 2009 13:43:43 GMT
Transfer-Encoding: chunked
Content-Type: application/octet-stream

Works OK in Firefox 9+ versions (9.0 beta, Aurora, Nightly): Kuva 030.jpg

filename*="UTF-8''Kuva 030.jpg" should be without quotes filename*=UTF-8''Kuva 030.jpg

Bug 704989 - add workaround for broken Outlook Web App (OWA) attachment handling