5330 has been fully supported since 4.0 UR5 for all models except Gigabit phones, only Gigabit phones require 5.0 UR2 (5.0.2.12+). There is no 5.2 software. Phones pull the firmware automatically at boot based on the TFTP information in either DHCP or statically assigned in the phone itself, you don't just update the sets firmware, it must match the PBX software.
The only place to get the SX-200 software, which includes the set firmware, is from Mitel Online and you must be an authorized dealer to access that site.