In RC1 (and also 584 of the daily builds) I'm getting a new tab opening when doing a Ctrl-click on emails to select them. This happens on both firefox and IE. This issue seemed to not be there in the daily builds before RC1 (or at least very close before release).
There is already a topic for this problem: http://roundcubeforum.net/forum/index.php?topic=1640.0