So I've been using Thunderbird for about 16 months now, and I finally decided to fix an annoyance. Anytime I get an email from someone using Outlook (often), any smiley faces they use in their message come across as the letter "J". It happened so often I kind of got used to it, thought of a J as a weird smiley. But that's not the correct behavior when something bothers you, so I found an implemented a fix I found. You can find the same info elsewhere, of course – a million monkeys and all that.
Go to your Firefox (you are using Firefox, right?) directory and find the res/fonts folder (on my computer it was at C:\Program Files\Mozilla Firefox\res\fonts).
Use wordpad to open the file fontEncoding.properties.
After the line "# Symbol font" add this: encoding.wingdings.ttf = windows-1252 encoding.wingdings2.ttf = windows-1252 encoding.wingdings3.ttf = windows-1252 encoding.webdings.ttf = windows-1252
Now copy this file (or the entire directory) into your Thunderbird install directory under …\res\fonts (for me, C:\Program Files\Mozilla Thunderbird\res\fonts). There wasn't a fonts directory there before, so I had to create it.
Then restart Thunderbird. Test by opening a file that had included a J, or just get someone running Outlook to send you a smiley. 
Update on 10/29/08
So it turns out the latest version of Firefox (the fantastic Firefox 3) does not include the fontEncoding.properties file. Bummer!
I looked at the file I have and the copyright/license seems to indicate it can be distributed for free. Working under that assumption, I've made it available here.
YMMV, don't take any wooden nickels, this might blow up your computer, use with care, etc.
Download fontEncoding.properties
Enjoy!
Update on 11/12/10
Lenin, below, posted a link to a fix. Of course I accidentally marked that as spam and poof it was gone. So here's what he found:
