How can I change the default encoding type Thunderbird uses when composing a new email?
In Thunderbird 78+:
Menu > Options > General > Language & Appearance > Advanced button > Text encoding
Tools → Options → Display → Formatting → Fonts → Advanced... → Character Encodings → Outgoing Mail:
For Thunderbird 17 on the Mac, you can find this setting at:
Thunderbird > Preferences > Display > Formatting > Advanced
Look at the Character Encodings section, as seen here:
On Mac OS X you have to use the Config Editor (also know as about:config) found in Preferences -> Advanced -> General. Change the value of the preference named mailnews.send_default_charset to UTF-8.