Middle-click is used for Paste in Ubuntu. All you have to do to Copy is select any text and then Middle-click to Paste.
There must be a way to change that behavior. Perhaps you could even do it without going into the source code and re-compiling.
Do you really want give up the middle click paste?! Middle-click paste is one of the greatest conveniences of X desktops. It saves me a lot of time and I believe it kept me from getting the Carpal tunnel syndrome.
While tiring to figure how to get the closing-tabs-by-middle-clinking behavior, closing tabs one can close tabs by pressing Ctrl-w.
Edit: Sorry, I didn't realize that the question author was talking about middle-clicking ON the tabs to close them. Both functionalities can co-exist.