I’m afraid this is still not officially supported, and covered by a rather [https://bugzilla.mozilla.org/show_bug.cgi?id=195012 old bug]. Not a real surprise, since Excel is a Microsoft product and hence such feature may be able to work only for e.g. Office or Outlook, but probably causing incompatibility issues with other clients. You may however find a workaround [http://forums.mozillazine.org/viewtopic.php?f=39&t=595940&st=0&sk=t&sd=a here], and copying to Word first and from there to Thunderbird should do as well. There is also an add-on called [https://addons.mozilla.org/thunderbird/addon/dafizilla-table2clipboard/ table2Clipboard] someone claimed to be successful with, but I’m not sure if that still works or is still supported. Can you report back?