allen has an excellent and constructive evaluation on the usability of the default Thunderbird theme (Qute) icons in this thread at MozillaZine forums.

Screenshot of Thunderbird Qute theme icons

Interestingly enough, I've used Thunderbird since before the current default theme was instituted and never noticed the usability flaws of some of the icons before I read allen's post. I tend to read the text labels, rather than decipher the meaning of an icon. Nevertheless, I can see fully how the icons are confusing and don't convey the true intention of their actions - a new user could be slightly addled without the assistance of text labels.

Well, it all goes to show how little benefit I've derived from the HCI module I'm taking this semester.

Note: No Qute theme-bashing, please (and none ever intended) - Qute is actually a theme that I really like, in fact.