Rated 5 out of 5 stars
This is one of the best add-ons I'm using.
To make it work for Thunderbird 5 just download the package dispmua-1.6.6.xpi from the website http://www.juergen-ernst.de/addons/dispmua.html then open the file with WinZIP/WinRAR - do not extract it - open for editing the file install.rdf and on the bottom of it replace the line
Then exit the archiver. That should save your changes.
After that install the add-on manually by going to Tools/Add-on menu and press the [Install] button. Choose the just modified XPI package.
After that you can upgrade to TB5.
Should work just fine.