One solution is to use Logitech mouse configuration software to map middle click to cmd+click, but there are some problems with this. Obviously, this will do you no good unless you have a Logitech mouse. Even if you do, this will screw up other applications that use the middle button properly. It only gets you half way there, anyway, because a cmd+click won't close a tab.

For a long time now, the nightly builds have included the correct functionality, but if you're not working on Firefox development, it's too much work to keep up with these, and they could have more bugs and problems than they're worth.

I've finally found a solution in Deer Park Alpha 1, the alpha release of the "next generation" Firefox. Yes, it's alpha, and it's for "testing purposes only", but over the past few weeks of daily use for me I've noticed very few, if any problems. A couple extensions don't work with the newer version, but it's a small price to pay for finally getting my middle button back.

Download the Mac disk image, mount it and copy the Deer Park application to your Applications folder. Be sure make a backup copy of your home/Library/Application Support/Firefox directory in case your profile gets corrupted, or you need to switch back to Firefox 1.0x.