Rated 5 out of 5 stars
If you need this functionality for Thunderbird 68, try the SmartTemplate4 Add-on, which now also includes similar functionality to the also abandoned Stationery Add-on.
You can add templates to the write/reply/forward buttons on ST4 Settings, under Accounts: "File Templates". On top of that, you can use variables, such as %from(firstname)% and insert dates, change subject lines, add recipients etc.
Rated 4 out of 5 stars
It is much faster than using the templates in TB (because my folder panel is very long) and it does more (with MultipleTemplateLoader, you can use templates for "Reply" and "Forward to").
But when I'm only composing a new mail, I miss the ability of an internal TB template to fill in the From: field, the To: and the Subject.
Half of my use of the templates is not in reply to a mail, but as an initial discussion.
Where can I put some code in the HTML file of an external template to have "From:", "To:" and "Subject" predefined?
Or would it be possible to include internal TB templates in the list?