[Kindle Fire HDX 7" & 8.9" Q&A, Help & Troubleshoot] draxie:
Here's the diff for the firmware images:

[COLOR="Purple"]HDX #[/COLOR] diff -rqs update-kindle-14.4.5.5_user_45500*/images
Files update-kindle-14.4.5.5_user_455001320/images/NON-HLOS.bin and update-kindle-14.4.5.5_user_455002120/images/NON-HLOS.bin are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/emmc_appsboot.mbn and update-kindle-14.4.5.5_user_455002120/images/emmc_appsboot.mbn [COLOR="Red"]differ[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/fwupdate.tar and update-kindle-14.4.5.5_user_455002120/images/fwupdate.tar are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/modem_update and update-kindle-14.4.5.5_user_455002120/images/modem_update are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/rpm.mbn and update-kindle-14.4.5.5_user_455002120/images/rpm.mbn are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/sbl1.mbn and update-kindle-14.4.5.5_user_455002120/images/sbl1.mbn are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/sdi.mbn and update-kindle-14.4.5.5_user_455002120/images/sdi.mbn are [COLOR="Lime"]identical[/COLOR]
Files update-kindle-14.4.5.5_user_455001320/images/tz.mbn and update-kindle-14.4.5.5_user_455002120/images/tz.mbn are [COLOR="Lime"]identical[/COLOR]

Since aboot isn't used when re-packing, this looks good.

As for aboot, the code is indeed identical, it's only the very first certificate

(which is *not* the one that's expired, but the one that generates the signature)

and consequently the signature itself are different (but the hash that's being

signed is the same, since the code is the same).

[COLOR="Purple"]HDX #[/COLOR] mkdir {13,21}20
[COLOR="Purple"]HDX #[/COLOR] cd 1320/
[COLOR="Purple"]1320 #[/COLOR] parse-aboot ../update-kindle-14.4.5.5_user_455001320/images/emmc_appsboot.mbn
image size: 358364
code size: 351964
signature size: 256
certificate chain size: 6144
c.0: C = US, ST = California, L = Sunnyvale, OU = Production Kernel Key, OU = Lab126, O = Amazon, CN = Lab126 Root CA 1
error 10 at 2 depth lookup:certificate has expired
OK
0000000 0d 1d 34 44 81 36 de 2f 48 53 98 8f bc a3 da 83 >..4D.6./HS......<
0000016 90 a2 ba c1 a4 e2 f7 77 41 e7 80 01 2a 19 7e 2c >.......wA...*.~,<
0000032
parse-aboot: terminating
[COLOR="Purple"]1320 #[/COLOR] cd ../2120/
[COLOR="Purple"]2120 #[/COLOR] parse-aboot ../update-kindle-14.4.5.5_user_455002120/images/emmc_appsboot.mbn
image size: 358364
code size: 351964
signature size: 256
certificate chain size: 6144
c.0: C = US, ST = California, L = Sunnyvale, OU = Production Kernel Key, OU = Lab126, O = Amazon, CN = Lab126 Root CA 1
error 10 at 2 depth lookup:certificate has expired
OK
0000000 0d 1d 34 44 81 36 de 2f 48 53 98 8f bc a3 da 83 >..4D.6./HS......<
0000016 90 a2 ba c1 a4 e2 f7 77 41 e7 80 01 2a 19 7e 2c >.......wA...*.~,<
0000032
parse-aboot: terminating
[COLOR="Purple"]2120 #[/COLOR] cd ..
[COLOR="Purple"]HDX #[/COLOR] diff -rqs ??20
Files 1320/c.0 and 2120/c.0 [COLOR="Red"]differ[/COLOR]
Files 1320/c.1 and 2120/c.1 are [COLOR="Lime"]identical[/COLOR]
Files 1320/c.2 and 2120/c.2 are [COLOR="Lime"]identical[/COLOR]
Files 1320/cafile and 2120/cafile are [COLOR="Lime"]identical[/COLOR]
Files 1320/code and 2120/code are [COLOR="Lime"]identical[/COLOR]

As for the reason for the update, I don't think it has to do with aboot.

I guess it's just various fixes (including libstagefright_*).