Title says all 
Is possible to that with Directory Opus?
I'm pretty addicted to middle-clicking --> new tab while browsing with Firefox.
So I'd like to do the same on tree folder view: middle click on a folder and that should be opened in a new tab on the current lister.
You can do it with a middle double-click, under Settings -> File Types -> All Folders -> Events. The command would b something like this:
Go FROMSEL NEWTAB=findexisting(If it doesn't work, try removing the FROMSEL argument. I'm not able to easily test it as I write this.)
Oops, sorry, I overlooked that you wanted it to work in the tree. I don't think there's a way to change what middle-clicking does there at the moment.