Rated 5 out of 5 stars
This is a nice add-on, it works perfectly whenever I need to redirect e-mails.
As mentioned previously, you have to download the latest version (for Thunderbird > 2.x) from http://downloads.mozdev.org/mailredirect/mailredirect.xpi (see http://mailredirect.mozdev.org/ for more information). The current version (Mail Redirect 0.7.6.4) is officially compatible with Thunderbird 3.0 - 5.0.
Workaround for Thunderbird 6.0:
Take the latest mailredirect.xpi from mozdev.org (see above), unpack this zip archive and edit the file install.rdf from this archive. In between the lines surrounded by the tag "<em:targetApplication>" find the entry below the comment "<!-- Thunderbird -->" and change the line
Pack the files again into a zip archive named mailredirect.xpi and install this from Thunderbird. This works for me, without much surprise, as Thunderbird 6.0 is mostly a security update. Of course, I cannot guarantee that nothing goes wrong when simply changing the maxVersion without adapting the add-on.