F5 has used the Linux standard MD5 hashed passwords since the introduction of V9. These passwords are resistant to rainbow tables, but not completely immune.
Recently, BIG-IP 11.4 was enhanced to hash local passwords with the salted SHA512 algorithm rather than MD5. These hashes start with $6$<salt>$<hash> and are considered much more resistant to rainbow table attacks.
F5 is always looking for ways to make our platform more secure.