The usage of "therefor" looks trite and legalistic to me, but it's an acceptable spelling for "therefore." You can go on writing "therefor" if you're really comfortable with it, but you've got to be consistent. Don't allow the members of your association's Board of Trustees to catch you inadvertently spelling it "therefore" one day, for they might think that you've been using "therefor" simply as a manifestation of your contrarian or lawyerly streak on matters of English usage.
