I was problems with Firefox on Debian Squeeze and webcalendar over Apache with name-based SSL virtual hosts.
A miserable solution... I disabled the check in includes/functions.php file.
See the changes (diff):
----------------------------------
--- functions.php.orig 2012-03-09 00:45:06.661836281 -0300
+++ functions.php 2012-03-09 00:41:07.841834980 -0300
@@ -6118,7 +6118,7...
2012-03-08 19:50:31 PST in WebCalendar
