The Simplest Way to Make Thunderbird Templates

Without even once hitting the Advanced button!  

You'll need a graphic with a very simple pattern so it'll be easy to read text over the top of it.  Use one like this:  

Start a new message via File - New - Message (don't click the Write icon).

Once you're in the new message window, click inside the text area (where you write the body of your email) so that the Format menu will become available.  Then click Format - Page Colors and Background. 

 

Use Choose File to browse to your background file.  

Click OK.  Now the background should tile all over the message window. 

In the Subject line, give it a name to tell you what's in the Template (since there's no preview of stats like in OE)

File - Save As - Template

And there she is, in the Templates folder.  Just double-click on the subject line to start a new message using that as the stationery. 

Tip:  don't move Templates from one Template folder to another.  Leave them in whichever Template folder they went into naturally, possibly the one under Local Folders if you ever used Global Inbox.  Don't know why, but moving my Templates messed them all up and I had to start again.