Works for Firmware >6.00 if... Rated 5 out of 5 stars

... one tiny change is made to the version posted at

Open the xpi-file (which is an archive) and open the included file "install.rdf". The line "<em:maxVersion>17.0.*</em:maxVersion>" needs to be adapted to at least the version of Thunderbird you are running. Or you set it to an incredibly high value like "<em:maxVersion>50.0.*</em:maxVersion>". Uninstall an existing version of Thunderbox and then re-install the new adapted version and it works again ;-)Greets