e3 flasher or progskeet is your best option, install otherOS++ and run QA flag extra .pkg to enable QA Downgrading (this stops your hard drive data being deleted when you downgrade) then update sign in and sync your backup trophes when you finished playing online or want to be back on 3.55 press downgrade on your flasher.
once finished reboot the console and it will say connect a controller put your 3.55 PS3UPDAT.PUP on a USB stick or external hard drive in the normal PS3/UPDATE folder then connect the controller it will still say install a 4.11 firmware but you can install any firmware you like as long as your ps3 supported it in the first place.