This reply just came back from Frank Hebblethwaite:
I don't have an answer for your question at the moment. Give me a couple of days, and I'll see what I can come up with.
In the meantime, I would suggest going to the Martin Luther King Library (the main DC Public Library) and looking for a listing for Ford's Theatre in the 1865 Washington City Directory. The city directory should either give you the exact street address for Ford's Theatre, or since it was a prominent, commercial structure, it might just say, "10th Street, between E & F." In other words, it may not have had a specific, numbered address.
Now that you have roused my curiosity, please let me know if you find the answer before I get back to you.
Jim Garrett - you and I were discussing the address this morning. Can you slide over a few blocks to the MLK Library and check the 1865 City Directory? I'm going to bet on it saying "Tenth Street between E & F."