From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

Thunderbird doesn't provide a way to add more emoticons to the "insert a smiley face" list box used when composing a HTML message. You need to embed a .GIF file for the emoticon/smiley face in the message. However, the Smilie Inserter and mailTagger extensions for Thunderbird make it easier to do that. For SeaMonkey a modified version of mailTagger is available.