If you click on the small gear and then choose Rearrange Gadgets, then you can move the page tree to the other side. But I have noticed that EPiServer doesn't work that well anymore with Firefox since an update some weeks ago. So maybe try this in IE or Chrome.
Can someone please explain what just happened? For some reason the page-tree now appears in the asssets-gadget to the right instead of it's own gadget to the left. I'm not able to add any gadgets at all in the left panel. No error-messages at all, and exceptions are thrown