How to protect furniture in a leaky garage?
Just put the furniture up on blocks.  Simple blocks of cheap wood.  "1 inch" (meaning 3/4") strips are fine. 

You don't need to avoid water ever contacting the furniture, but you do need anything that does to dry out quickly.  For that reason, wrapping in any form of plastic is likely harmful.

Now drape some plastic loosely over the top, or a cheap tarp, making sure there's plenty of airflow.  Done.