Update: Since version 1.5.1 Thunar supports multiple tabs. Here's how to install it.
If you're wondering when will Thunar support tabs, well the answer is never (see the reference bellow).If you really want/need a file manager with tabs support for your Xubuntu, you could easily install nautilus:
$ sudo apt-get install nautilus
|Nautilus will add tabs support for Xfce/Xubuntu|
When will thunar support tabs? (Xfce Forum)