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.

When you add a emoticon using the "insert a smiley face" list box its actually stored as smiley face (text) characters in the message. Thunderbird displays any smiley face characters as a graphics image while other email clients display the smiley face characters as is. This is a built-in feature enabled by the mail.display_glyph setting and why what you see and other users see may differ. However, if you embed a .GIF file using one of the add-ons everybody sees a graphics image.