I think historically "therefore" and "therefor" are the same word, if one looks at the meaning. I also agree that "therefore" is the preferred, possibly the correct, version even in cases where "therefor" is used like this:
...
"on receipt of the goods the buyer shall effect immediate payment therefore"
...
although that certainly does not follow the usual pattern: thereby, therein, thereafter, etc.
Mmm .. I wouldn't say that was "the preferred, possibly the correct, version", boozer. Faced with your sentence, I'd definitely correct the "therefore" to "therefor".
That said, I could probably count on the fingers of one hand the number of times in my life I've used "therefor", whereas I use "therefore" quite a lot. I'm sure you're right that historically they were the same word; now, however, they're quite distinct.
PS Actually, thinking about it, I even pronounce the two words differently -
therefor with the stress on the second syllable,
therefore with the stress on the first....