Mozilla Thunderbird before Enigmail is installed (return to the Enigmail and Mozilla / Mozilla Thunderbird tutorial clicking here or hitting the "Back" button of your browser, if you come from here)

Mozilla Thunderbird once Enigmail is installed (Enigmail menu has appeared)

Mozilla Thunderbird Enigmail menu (Preferences highlighted)

Enigmail global preferences (with path for GnuPG on Windows™; on Linux it would probably be /usr/bin/gpg)

Mozilla Thunderbird Enigmail menu (Generate Key command highlighted) *

Enigmail Generate PGP Key dialog
It fills automatically your identity, and simply asks you for a passphrase and a comment

Enigmail Generate PGP Key dialog to confirm

Enigmail Generate Key Dialog (with output of key generation in progress)

Dialog telling you your key pair has just been generated

OpenPGP Preferences by account. It will find automatically the key pair(s) to use if the account(s) name and email correspond to the key pair(s) one(s).

Mozilla Thunderbird Compose: dialog
(with Enigmail available)

Mozilla Thunderbird Compose: dialog
(with Enigmail available, and Signature and Encryption selected, see bottom right corner)

This is a message we are going to send with our signature. Notice the little pen on the bottom right corner is colored in green, that means it's going to get signed before it is sent.

Here is what the passphrase asking dialog looks like. Note the duration you will be able to sign and decrypt stuff without having to type it in again, is configurable through Enigmail global options.

This is what an signed message looks like, if you know and trust (i.e. have verified and signed this key through GnuPG command line or a graphical front-end like kgpg or WinPT) the signer's identity. We can see the mail is signed looking a the yellow pen symbol on the right (a right click on it will tell you even more). If you don't know yet a signing key, Enigmail will offer to look it up in one of its configured keyservers, but you will of course have to be sure (and to concretize this trust signing it with your private key) the public key you got corresponds to the owner it says, before you can trust its signatures!

This is what a signed message really is: simply the original message, with a text armor around it that contains and defines the span of the signature --though Enigmail will show you this signature in a more appropriate, nicer form (see previous image).