Installing extensions in Thunderbird is a little bit different than Firefox. Since Thunderbird does not have a built-in browser, you cannot just navigate to a site, click on an .xpi and have it automatically install itself. For the same reason, you can't attempt to download the Thunderbird .xpi with Firefox because Firefox would try to install it in itself. To get around this, many times the .xpi is available as a .zip instead (since an .xpi is nothing more than a renamed .zip file anyway).
To make a long story short, just rename TEBEbeta(20090105_115843).zip to TEBEbeta(20090105_115843).xpi and install it through the Thunderbird add-on manager - Tools > Add-ons > Install....