[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] br0adband:
I just got an HDX 7 (3rd gen) today for free, been mucking around with it for the past few hours (it's a piece of crap, really) and kinda pissed at Amazon (nothing new for me considering).

Here's what I've gleaned so far in a few hours of research... and please understand that since the OP said he's already on 4.5.3 this info is specifically related to what can be done with an HDX running 4.5.3.

What you can't do: If you're on 4.5.3 forget doing anything at all - you can't root it, you can't install custom ROMs (no Safestrap use at all), you can't unlock the bootloader, you can't do a damned thing, period. Well, you could use it like Amazon wants you to use it: a consumption device overall but that's not why we're here at XDA basically so there goes that ridiculous idea. :) (note this information applies to 4.5.3 specifically and nothing else)

What you can do: The only thing you CAN do if you're on 4.5.3 is to roll it back to 3.2.8 using the 3.2.8 firmware bin copied to the storage root (you can find that yourself someplace, I did but I don't have the particular link to that particular thread at the moment).

Once you're on 3.2.8 you CAN root it (I don't have the thread for that at the moment, I think the HDX Toolkit or whatever can do it using Towelroot or something). At that point you can stick with 3.2.8 and do whatever (disable OTA's, etc) - I believe you can install Safestrap as well and then install that Nexus ROM I see mentioned but that's Jelly Bean based and CM11 and CM12/12.1 cannot be used presently - the best you'll ever be able to do with 3.2.8 is running the Nexus ROM as I understand things (I could be wrong on that, someone else can chime in as needed).

Another thing you can do once you're on 3.2.8: you CAN update it to 4.5.2 using the 4.5.2 firmware bin copied to the storage root (again, you'll have to find this yourself) and 4.5.2 CAN be rooted but that's where you're stuck because you can't install Safestrap or any custom ROMs at all (not even the Nexus one), but you can kill off the OTA updates and stay with 4.5.2 as long as you want because if you don't it will update to 4.5.3 again the first chance it gets.

I didn't know all this about 6 hours ago, I got the HDX and it had 3.0.5 on it - I didn't know not to update it - well, yes, I did, but I forgot about the WiFi thing and as soon as I realized that the update had downloaded it was already too late - as soon as I rebooted it installed and I couldn't stop it from happening so that ruined my ability to do the bootloader unlock and potentially get CM12.1 on it. My fault, my mistake, but that's what happens sometimes.

As it stands I don't even want this piece of crap to be honest, I find it useless overall unless you can do custom ROMs 'cause Amazon and FireOS just flat out suck for me - if I can't do what I want with the device then I can't use it, basically, so it's listed on craigslist for sale at the moment and I hope someone grabs it quick.

That's the gist of what I've learned in the past few hours with respect to this HDX device and doing an awful lot of reading and research not only here at XDA but MobileRead and some other places.

Hope this helps...