It's being worked on :)
see <http://bugzilla.mozilla.org/show_bug.cgi?id=22775>
Sol Hell wrote:
> Why does mozilla insist on not including middle (roll) button
> navigation. I mean if you click the middle mouse butoon and then you can
> navigate up, down, right, left, etc. I am not talking about turning the
> roll button.
> This is such a major annoyance, especially if you want to convert IE
> users. I am willing to make netscape my default but this is keeping me
> from it since it slows down my browsing experience.
>
