How to stop smiley in Thunderbird showing up as a J?

Here's my solution. I wrote a Thunderbird Add-on that replaces J, L, è, à, and · with the correct UTF-8 emoticon, arrow or whatever.

Before:

After:

Credit to Isxek for getting me on the right track though!

This Lifehacker article has some explanation on what's going on. What it basically says is that when the sender types in a ":)" character, their Outlook automatically converts it into a smiley rendered in the Wingdings font. As far as I know, this happens when Outlook uses Word to edit email messages.

At the moment, I'm on a box with Outlook installed, not Thunderbird. Is it possible for you to change the encoding to ISO-8859-1 to see what happens?