Don't believe any websites claiming that they can downgrade from 4.30 to 3.55, because it is no longer possible after 3.55, the only way to downgrade is by flashing the nand with a e3 flasher and that is very limited to what fw you can actually downgrade to.
I have 2 ps3's, one is at a really old firmware (before 3.55), is it possible for me to downgrade the other one that is on 4.30? (I swear there was method of downgrading, connecting 2 ps3's by ethernet)
AFAIK, it's currently impposibru to downgrade a single PS3 which is on 3.65+. IIRC, you will need another PS3 running 3.55 or lower firmware in order to downgrade the PS3 running the later forum. If you search around the forums, I'm sure you could find quite a few guides showing you how to do this.