You're doing it wrong. Unlike TWRP 2.0.0RC0, 2.1.0 is not supposed to be loaded into boot and your system is probably panicking. You need a bootloader first.
First you need a bootloader with recovery selection. There are several to choose from at this point.
I would recommend FIREFIREFIRE 1.3
Download one of them and extract the bin somewhere. Lets say c:\firebuild\
Next we need a recovery partition, you already have that in the twrp-fire-2.1.0.img, so you should be set. Copy that to c:\firebuild\ as well.
Now, lets make sure you have the andriod sdk in your path. This way you don't have to be in the C:\Program Files (x86)\Android\android-sdk\platform-tools folder to run commands and your images can be elsewhere.
1. right click on computer and select properties.
2. click on advanced settings on the left of the window.
3. click on environment variables at the bottom right of the window.
4. in the system variables section scroll down until you find path, highlight it, and select edit.
5. paste the following at the end of the line.
;C:\Program Files (x86)\Android\android-sdk\platform-tools;C:\Program Files (x86)\Android\android-sdk\tools;
For example, my path is:
C:\Program Files (x86)\NVIDIA Corporation\PhysX\Common;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\;;C:\Program Files\Lucidlogix Technologies\VIRTU;C:\Program Files (x86)\QuickTime\QTSystem\;C:\Program Files (x86)\Android\android-sdk\platform-tools;C:\Program Files (x86)\Android\android-sdk\tools;
Now that your path is set, lets go into the command prompt.
First lets change the directory to c:\firebuild\ :
In this folder we should have u-boot.bin and twrp-fire-2.1.0.img.
First lets load your bootloader.
fastboot -i 0x1949 flash bootloader u-boot.bin
and start your device.
I would recommend rebooting after flashing this, so run
if it does not reboot on its own.
Now lets run
fastboot flash recovery twrp-fire-2.1.0.img
Note: Because we have the bootloader now the -i 0x1949 is no longer needed.
After running the above, the kindle should automatically start into recovery.
From here you will be able to mount it as an external drive and copy over any rom you would like to use.
Pick something from here Kindle Fire Development *FULL LIST* - 17.04.2012 -
under the Custom / Stock Roms section.