Personally I think it is bad email etiquette to send without a subject and I delete any messages without one because they are usually spam or malicious.

There are several add ons that claim to allow you to do this. Go to the add on page and search for No Subject and see if anything there helps you.

https://addons.mozilla.org/en-US/thunderbird/

Thank you for your help. I got the add-on but then got this message: This can not be installed because it is not compatible with Thunderbird 24.5.0. I guess I've got the wrong version of Thunderbird.

When choosing add on always look at the fine print under the name to see what versions they are compatible with. Some are no longer maintained and only work with older versions.

Thanks again. I'll check that out.

Try this one:

https://addons.mozilla.org/en-US/thunderbird/addon/sendwithoutsubject/

although I agree the practice of sending with no subject should be discouraged except in unusual cases, such as the one mentioned in the 2nd review..