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.