I'm glad you posted that your version of Thunderbird did spell check. 

Every time I checked for updates mine said it was the latest version.  However Version 9 must be different because I could never get any spell check to work.  That led me to the Mozilla download site and download Thunderbird again.  However the version on the Mozilla site was 38.5.0

I took a chance and downloaded it, installed it and it apparently it imported all my emails without problems and it does spell check.

Thanks for the post.