2014-07-23: Mozilla has finally seen the light and re-enabled middle-click to close the last tab in Firefox! Probably not because of the massive install base of this add-on (454 as of this writing), but because the resolution to bug 997681 (Bugzilla; see also 865826) has resulted in the re-introduction of the close button on the last tab. That, in turn, is probably due to "peer pressure" from Chrome (and maybe IE)! It's still not enabled for Thunderbird, though, so it looks like version 0.1.0 last month was just in time.
As of 0.1.0: requires Firefox 21 or higher.
As of 0.0.5: requires Firefox 13 or higher.
As of 0.0.2:
The checkbox preference is not guaranteed to work in anything older than Firefox 12 (because the Add-on SDK is so new??). Core functionality, however, seems to work in Firefox 8 [Firefox 7 support dropped as of 0.0.4.1] and up - you'll just have to adjust the about:config preference by hand.
This extension merely restores the default behavior of Firefox 3.6 regarding tab-middle-click.
Firefox 4-7 get no love...