Since a recent *zypper patch *resulted in T’bird updating to v. 31.3.0 I noticed that my spell checker (comes on automatically at send) has been unable to find many common words and has suggested strange changes to others that I know to be spelled correctly (I’m an English speaking American). I drove me crazy for a while, but then…
I found that the default language had changed with the update and was now English - Australia. A switch back to English - United States did the trick. I don’t know if this affects only US users, so I’ll pass this tip along in hopes someone finds it helpful.
Thunderbird>Edit>Preferences>Composition>Spelling>Language