Well…
Something booted from something. FreeBSD is present on a drive .....
Btw : before the BIOS even knows it HAS drives in the system, it menu is accessible. The method of accessing has nothing to do with FreeBSD or pfSense. See the bios manual for the procedure.
When the USB booted (the pfSense pre installer) , AND it recognizes a disk where it can install TO, then the install menu will be shown. This menu will offer you to empty your SSD drive .
Of course, if these conditions aren't met, nothing will ne shown ... because nothing can be done.