Uhm, A dump of the bios is the information stored on the nor of ps3. It has everything that tells the ps3 to be a ps3. It also has very important, non replaceable per console data. If anything goes wrong in the downgrade process, you will need this bios dump to fix your ps3. You should also have some alcohol to remove all of the dust to avoid bad contact errors. To validate a dump of a bios you have to download e3 10-28 update.bin file from e3 tech.net, put it in the root on your sd card put it in the e3 flasher and set the switches at 1 and 2 down and the rest up. Turn on ps3 if all your lights lights up, your e3 flasher has taken the update. Remember to press reset, after that move the switches to 1 and 2 down, 3 up, 4,5, and 6 down, press reset and after that press start and wait for the blue led light to light up in sequence. When all lights have lit and alternate flash the backup is done.
Repeat these steps 2-3 times in-case some of the backup is wrong (Usually if 1 is wrong all of them are).
Download HXD editor and look at your backup:
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if the 00's between 18.71% - 29.01%
And if the FF's is between 10.42% - 10.48% you have sucsessfully made a good backup of your bios. If you don't understand how to find out if your bios is valid or not, you can always post it here

But I think ps3hax is more into that stuff.
I hope you appreciate this I used a lot time of writing this shit to you.