Rutger,
I accidentally installed the M92 firmware on my i62HD and it booted up funny too and I was able to recover from this.
First, I think you're not using the correct version of the firmware.
I recommend you go to the first discussion thread in this forum which contains links to the latest version of the firmware and be sure you've downloaded the correct version for your firefly. I would not get the software from
www.onyx-international.com since it's different. You can download the firmware from the forum or from
www.ereader-store.de. I've installed the version from the forum and it worked.
You want to copy the firmware file to an empty MicroSD card and keep it in the root folder. Do not create any folders on the MicroSD card or include any other files. Power off the i62 HD Firefly and insert the MicroSD card into the card reader slot. When you power on the i62 HD Firefly hold down the joystick button while turning on the power. Release the button once you see the updating firmware message. The correct version of the firmware should install and your i62 HD Firefly should boot up properly.