When you buy the SSD as an upgrade, you will need to install the boot sys and image it.
(Note - If you are replacing a failed SSD in a 5545 or 5555, it will get the software from its mate SSD since they are in a RAID pair.)
You can follow this procedure to install an image on a fresh SSD:
http://www.cisco.com/c/en/us/td/docs/security/firepower/quick_start/reimage/asa-ftd-reimage.html#pgfId-146378
