The built in templates are not too well suited to this. Can I suggest you try the Stationery add-on which will remember and apply a template and allows you to switch between alternatives. You'll need to convert your templates to html files. Just create a new empty message using a template and use File|Save As to save it as an html file. Then select that html file in Stationery. I'd suggest you create a specific folder just for your Stationery templates. One big advantage is that you can use your stationery templates in any account, unlike thunderbird templates which are linked to the account in which they are defined. One word of caution with Stationery. If you have started writing and change the stationery then the material you have typed will vanish. This can happen unexpectedly if you change the "From:" address. You can use a ctrl+a ctrl+c to copy everything you have written before making such changes, and then a ctrl+v to paste it back in.