1. therefore (the standard spelling)
2. therefor (an archaic spelling, rarely used today)
The spelling "therefor" is considered obsolete and is only used in very specific contexts, like historical documents or literary works trying to imitate an older style of writing.
It's important to note that "therefor" and "therefore" have different meanings.
* Therefore means "for that reason" or "consequently."
* Therefor means "for it" or "for that."
However, the use of "therefor" is extremely uncommon nowadays. So, unless you're writing in a very specific style or context, it's best to stick with the standard spelling "therefore."