It's the one dated 3-19. You might have already heard about it; an unreliable source posted about the firmware a little over a week back and I freely admit that they beat me to the punch. Here's how they described it:
We are lucky enough to get our hands on the firmware that was sent to us in our tester unit that gives you a fully English GUI and gives you root access for your Kyobo. This will allow you to install apps from the Amazon App Store and give you a ton of default e-reading applications.
I'm not being a jerk; they're 100% wrong. That firmware doesn't include any reading apps, nor does it include Amazon's app store. All you get in the hacked 3-19 update is Google Play and root access. It doesn't even come with Kindle or Aldiko.
I know this because I already had the firmware. I never posted on it because I didn't think it was worth the time to download and install. It will not let you install any apps. Since it also didn't come with any apps it was a frustrating waste of time.
What's more, my source with Kyobo Bookstore told me that particular firmware for the Kyobo Mirasol eReader is the only hacked firmware floating around at the moment. So any firmware you might encounter is probably going to be this one. He also confirmed that everyone else had the same issue I had.
I'm disappointed. I have wanted to turn this into an Android tablet ever since I got it. Sorry to burst your bubble, folks.