If you set up your preferences to “double click to activate items” and also use the “middle click: open folder in new tab” option, you will have to double middle click on a folder to open it in a new tab. This is not very convenient and contrasts with user expectations (users usually expect to have to perform double-clicks only with the left mouse button). The right-click action is unaffected: even with “double click to activate items”, you can open a context menu with a single right click (which is good).
I think the code relating to this is in "thunar-details-view.c" around lines 706-730, but I don't know exactly how to fix it.
Regardless if you choose middle click to open folder on a new window or tab, you'll need a double click. I don't know if users really think that the should be able to use the middle button with a single click, but judging by the preferences -> behavior dialog, it might lead them to think in this way. If you're still interested on hacking, if you remove the if @ line 719, you'll get what you want. But the result is a bit awkward, because if you open a folder, it's done so fast that you can't see the item getting selected. When opening a file, you get a small delay to open the app the handles this(unless you have a ultra fast computer), so you can see the file getting selected. Also note that you should hack thunar-compact-view.c and thunar-icon-view.c.
There is a patch provided in the other report. *** This bug has been marked as a duplicate of bug 10587 ***