Download

the most recent version of signature [download] should always be obtained from addons.mozilla.org

Install to thunderbird.

1. download the .xpi file [right click and save as the above link]
2. open thunder bird
3. open the extension manager (tools -> extensions)
4. click install button
5. browse to, and select the .xpi you just saved
6. say yes to the dialog that asks if you're sure you want to install
7. restart thunderbird

Using

the usage page details how to use this extension, from beginners to experts