how to make thunderbird use utf-8?

[Mike Wright]

Hi all,

This may be a bit OT but I know where the geniuses lurk.

Whenever I send an email using Thunderbird the following header is 
always included:

Content-Type: text/plain; charset=ISO-8859-1; format=flowed

How do I change it to charset=UTF-8?

I have climbed throughout about:config and changed/re-ordered everything 
that mentions ISO-8859-1 to UTF-8 and still all outbound mail contains 
the undesired header information.

I think that header causes Latin-Extended to show up as blobs with a ? 
inside; here's one: ¢  Should be a US cent sign.

Or am I looking in the wrong place?  Locale or something?

Anybody have any suggestions?

Thanks always,
Mike Wright