Enigmail adds Open PGP message encryption and authentication to your email client.
To the android person, this is the desktop plugin, for Android close all, see Quit Now, https://addons.mozilla.org/en-US/android/addon/quitnow/?
To insert addresses from your address book, enter some of the characters from the person's name in the To field.
A drop-down list will show all address book entries where the first or last name starts with the character(s) you entered would be displayed in the drop-down list.
The Development Channel lets you test an experimental new version of this add-on before it's released to the general public.
Once you install the development version, you will continue to get updates from this channel.