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I currently have a CEX Slim PS3 running 4.55 Rebug and use Multiman. Browsing on their site (http://rebug.me/) I find that they released a 4.70.1 COBRA REX. I don't have a Cobra Dongle, nor my ps3 is a REX, but they state that: "REX Cobra 7.1 EDITION – INSTALL ON CEX PS3 SYSTEM
REBUG 4.70.1 REX EDITION – APR. 2ND 2015"

I already run my PS2 backups with PS2classics manager and PS2 Placeholder (on PS2ISO Folder) and I intend to keep doing so.
Also, do I have to install rebug toolbox before or after the FW upgrade?
Is it safe to install this firmware on my CEX Slim? Thanks in advance.

This Firmware does not require a Cobra dongle because the system files from them are incorporated in the CFW.

You can safely install this CFW. It's the most stable CFW novadays, with great features. Also, the Toolbox should be installed AFTER the CFW installation.