From MozillaZine Knowledge Base

Symptoms

The body of your messages shows as strange characters that replaces your normal ASCII characters.

Solution

1. Exit Thunderbird.
2. Back up your prefs.js file.
3. Open prefs.js in a text editor (e.g., Notepad) and look for this line:

user_pref("font.name.monospace.x-western", "CSD10");

1. Note that "CSD10" may not be the name of the font in your prefs.js file.
2. Remove this line and save prefs.js as text.
3. Start Thunderbird and test it.