But there's a small and obvious workaround that I hope you haven't thought of yet (I'm full of unrealistic beliefs).
To check the spelling of your subject line in Mozilla Thunderbird, Netscape or Mozilla:
- Copy the Subject to the body.
- Click Spell.
- Alternatively, select Options | Check Spelling... from the menu.
- When the spell checker has finished, cut the subject line text from the body and insert it back to the Subject: field again.