the difference appears to be in pronounciation and how they modify a verb. therefor(accent for) and therefore(accent there).
therefor= for that, for it
therefore= for that reason, because of that
both are adverbs which define an action.
i purchased a jacket and recieved a receipt thereFOR. (for it)
one plus one equals two, THEREfore, two minus one equals one. (for that reason)
LB, that irritates me too. i use them correctly most of the time, but i also hate to type accent marks so i just run them together into one word

for example:
were=we're
theyre=they're
its= its and it's
i think its laziness