I recently subscribed to .mac, and to my surprise I wasn't able to publish calendars with Sunbird using the URLs provided by iCal or .Mac support. So I fooled around with Transmit and iDisk for a while and discovered that calendars can be accessed/published via Sunbird using the following "hidden" URL:
http://idisk.mac.com/accountname/Sites/.calendars/calendarname.ics
If only iCal could subscribe and publish simultaneously for a given calendar...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050525011340458