As mentioned by others, this addon does not work with Thunderbird 38 due to an encoding issue. I've looked at the code, and presented the trivial change needed to restore read/write capability of template files. See http://mesquilla.com/2015/07/15/fixing-quicktext-addon-for-thunderbird/
I've contacted the author asking that this change be done, or someone else released to do the change.
R Kent James
Chair, Thunderbird Council