When clicking OK and reopening the configuration dialog the server address was always changed to "http://localhost/owncloud/apps/calendar/caldav.php/calendars/admin/" (regardless what URL I set before).
One also can't edit anything, since all inputs are disabled (greyed out).
Therefore the only possibility to point to the correct URL is to change the settings file manually (which works but is a very tedious job).
I'm using RemiderFox 2.1.2 with Firefox 25.0.1 on Windows 7 x64.