You can choose the default opening mode for pages.

The default is to open pages in a multi-page view.

This is a personal preference for each user. Find the preference on the Opening sub-tab of the Extra tab of the Fred Preferences dialog box:

Should you want that the default opening should be in separate tabs, and you don't want that every user has to tick the Double-click Opens a New Page in Separate Tab, create an override sub OnPageDoubleClick in the CustomFredAddIn, and replace in it AddDesigner with OpenDesigner.

 <ShellCommand(CanExecute:="HasSelectedObject")> _

Public Sub OnPageDoubleClick()

  'If shift is pressed open object else open designer:

  If AddInUtils.IsShiftKeyPressed() Then

     EditObject()

  Else

     AddDesigner()

  End If

End Sub