Bad ROM flash - Kindle Fire stuck in boot
There are the two things of note that I see in your data:


The instructions indicate that you should use fastboot -i 0x1949 boot u-boot.bin. This is actually not correct, because the fastboot boot command is intended for hotbooting a boot.img, not a bootloader (they are different). The original XDA instructions give the proper command, which is fastboot -i 0x1949 flash bootloader u-boot.bin. This will install FIREFIREFIRE on your device, overwriting the stock bootloader.
Generally, if your KF hangs at the boot splash screen, it's stuck in fastboot mode. You can reset it to the normal boot mode by connecting it to a PC and executing fastboot -i 0x1949 oem idme bootmode 4000 and then rebooting. If you have FIREFIREFIRE installed you can leave off the -i 0x1949 flag because it uses a different vendor ID.