Therefore: serves as a conjunctive adverb or as a regular adverb. When it joins two clauses, it must be preceded by a semicolon and followed by a comma: The court upheld the lower court; therefore, the appellant lost once again. When it serves as a regular adverb, it needs no commas if you want to stress its modification of the verb: The court therefore ignored these arguments.
Therefor: a regular adverb that never joins clauses. It means “for it” or “for that thing or action.” It might appear like this: She earned millions for her company but was never compensated therefor.
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.