JUNOS prior to 5.0 was based on FreeBSD 2.2.6. JUNOS between 5.0 and 7.2 (inclusive) is based on FreeBSD 4.2. JUNOS 7.3 and higher is based on FreeBSD 4.10. JUNOS 8.5 is based on FreeBSD 6.1
That's all I can find about JunOS's FreeBSD version (not sure if it's even correct).
I don't see anything in Juniper documents about the corresponding FreeBSD version. Can anyone provide the FreeBSD version for newer JunOS releases up to 13.2?