User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20160123151951 Steps to reproduce: I selected a template to write a new message. Actual results: Japanese text is question marks in black diamonds. It means that the software doesn't know how to process the encoding. But I've been using these templates for a long time and didn't edit them! Creating a new template, making sure it is encoded as UTF-8 and saving it produces the same result. I guess UTF-8 is just not used when creating a message from a tempklate. Expected results: Well, I should be able to use the template.
Creating a new message from a message I have sent before is also broken the same way.
Creating a new message from a message I have just sent is also broken.
You reported the bug against TB 38. There is no support for this version any more. I've tried saving a template with Japanese text (これは長い日本語のテキストですので、行が折れると思います。) as UTF-8 template and editing the template as new message works perfectly in TB 45.1.1. So I cannot reproduce your problem. BTW: Editing a message as new, either template or other sent message, behaves the same, as you noticed.