You dont need to downgrade anymore. The only thing you have to do is to patch a perfect dump with 3 Musketeers patch and reflash it.
To check the dump you need three tools, or two as minimum. Get PS3 NOR/NAND Statistic and PS3 DUMP CHECKER V1.0 BUILD 4.88, also you can use a hex editor.
Drop the dump in statistic and you have to get a: "no errors found"
Drop the dump in checker (first activate:arrow on right corner - download lastest hashlist, to avoid a false "ros1/ros0 error")
You have to get a OK, but if you have one ros hash error,dont worry, the dump is safe anyway.
If you have some different errors from checkers, just reinstall the clip/linker.
After that you can open a hex editor and check some offsets to make sure the dump is not bad-if you want-but if you already got good results from the other tools, dont need the hex check.
After this you have to PATCH your dump to reflash it. Look for three musketeers patcher and follow the steps, you'll get a "bkpps3.bin.patched".
Save the original on your pc and rename the patched as the original in the microSD.
Insert in E3 flasher and reflash it.
Done.
You avoided all the heavy/boring process of downgrading a console.
When reset you can install a 4.80cfw.