Still you can make use of your message templates, you just cannot apply them to a new message you're already composing.
How to Use the "Templates" Folder in Mozilla Thunderbird or Netscape
To create a new message in Mozilla Thunderbird or Netscape from a template in the Templates folder:
- Open the Templates folder.
- Double-click on the template message you want to use.
- Alternatively, select Edit as New... from the message's context menu.
The Copy in the Templates Folder Is Not Affected
This creates a new message that looks exactly like the saved message in the Templates folder, but it is actually a copy of that message. You can edit the new message as you like, save it as a draft, send it, you can even save it as a new template. The original message in the Templates folder is not affected by any of these actions.