Yeah, the Condor Updater unfortunately doesn't work on FAT PS3 models it seems.
I also had issues when I was in 4.21 REX, although it's an amazing firmware. I decided to downgrade since I didn't need all the extra toys it comes packaged with.
I successfully downgraded using the 999 Downgrader, but while installing 3.55 Kmeaw again, I got a blue screen. My heart sank, but I let it sit there for 4-5 minutes.. nothing. I then plugged in my PS3 controller, and pressed X and surprisingly enough the screen responded, it went black then froze on me but I was able to shut it down by using the power switch on the back.
After I turned it back on, I noticed XMB booted back up. PHEW! But apparently Kmeaw didn't install, so I tried installing it again from XMB and lucky for me it worked. I swear I have no idea what happened, and I don't plan on doing another downgrade again anytime soon since I don't have a hardware flasher (just my nand backup).
Moral of the story is, I'd encourage you to wait until all the kinks are completely fixed. These CFW's are still new, and even though they've installed successfully for numerous people. There's still some bugs, look at Rogero's CFW for example. Unless you are in a serious hurry to play new releases, I'd seriously wait. I rather have a 3.55 PS3 than none at all.