Is there any addon or script that could remove this horrible "new" tree view in the Connect/Servers options, and replace it again with the normal and usable view from the older versions of mIRC?

what's so horrible about it? Perhaps if you gave us an idea of what it is that changed for you that you dont like, you'd get some better responses than the ones I know are waiting in the wings

I think it a vast improvement over the old dropdown menus.

i use no name script that u can customize the tree or use the regular. i dont know if its wat your looking for but its a thought.