An accidental therefor legitimately slipped past my spellcheck the other day. I meant to use therefore and, for whatever reason, missed typing the final "e." So what is the difference? Following are definitions from The Columbia Guide to Standard American English -
therefor: for or in return for that, for it - ex. I'll explain what we must do and the causes therefor.
therefore: consequently, hence, for that reason - ex. I don't have a key; therefore, I'll have to ring the bell.