The Send Format setting's guiding text says: "When sending messages in HTML format..." It applies only when you have composed a message in HTML.
The setting you are looking for is in Edit / Mail & Newsgroups Account Settings / <account name> / Composition & Addressing / Compose messages in HTML format.
If you need to ask more Mozilla related questions, there's also a special site for that,
MozillaZine Forums.
A hint: when asking questions, it usually helps the helpers if you tell the version of the software you are using.
Regards
Simon