|Select options from the three bar menu.|
|Select the Display | Formatting tab | Advanced button|
|Set outgoing mail text encoding to Unicode |
Select the "When possible, use the default text encoding in replies"
Click on Ok.
If that does not do it for you you will need to use the config editor to modify the setting mail.strictly_mime to true to force Thunderbird to use 7bit encoding in email. This is not an optimal setting but is sometimes needed to workaround the various Yahoo issues.