if your on 3.40 you can just install 3.55 official firmware, create a folder inside your usb flash drive and name it PS3 and then inside the PS3 folder create a new folder and then name that one UPDATE and then download this file:
https://mega.co.nz/#!NVwRGIQS!Pml4diGNhW3aMJe1mnSirW_66gAZKZnWo6cmqKMDvqU and put it inside the UPDATE folder and then in the XMB go to Settings > System Update > Update via Storage Media and then you can start installing the 3.55 firmware.
and then once your on 3.55 you can then install any custom firmware like Rebug 4.84.2 straight from there, by downloading it's UPDAT.PUP file and put it in the same UPDATE folder like before and then go to Update via Storage Media again.