The Browser Open URL in New Window action should open your link in a new tab (depending on your browser's settings). I don't think that opening in a new tab can be forced by Javascript if the default in the browser is set to open a new window. You can, however, force a new window to open....
If you want to have some control over the new window that's opened then you could use the Browser.ExecuteJavascript:
To control how new window is formatted:
"window.open('http://www.w3schools.com/jsref/met_win_open.asp', '_blank', 'toolbar=yes,scrollbars=yes,resizable=yes,top=300,left=300,width=400,height=400');"[/code:263grg0m]