[Amazon Echo] j10hx40r:
Here are some OTA urls for Echo Dot 2nd gen (codename - biscuit).

Fire OS 5.5.0.3 (564196920)

https://d1s31zyz7dcc2d.cloudfront.net/1b9718ec2da663bb299676df977055c9/update-kindle-full_biscuit-272.5.6.4_user_564196920.bin

Fire OS 5.5.3.1 (641575220) - Contributed by Lemon86

https://d1s31zyz7dcc2d.cloudfront.net/8811a0fc982bf3331dc54f5aec45d936/update-kindle-full_biscuit-272.6.4.1_user_641575220.bin

FireOS 5.5.4.6 (666694320) - Contributed by Lemon86

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/0cdf0a5782c43290ba9a598e1ede98e6/update-kindle-csm_biscuit-272.6.6.6_user_666694320.bin

Fire OS 5.5.4.9 (672720020) - Contributed by frostworx

https://d1s31zyz7dcc2d.cloudfront.net/60556d35239d902637f0fc503411b791/update-kindle-csm_biscuit-272.6.7.2_user_672720020.bin

Fire OS 5.5.5.0 (673726720)

https://d1s31zyz7dcc2d.cloudfront.net/f4994d8b2b10442d42e07b8c5d996a91/update-kindle-csm_biscuit-272.6.7.3_user_673726720.bin

Fire OS 5.5.5.4 (680767620)

https://d1s31zyz7dcc2d.cloudfront.net/47a1457e0802980eb32f63cd3ce355c0/update-kindle-csm_biscuit-272.6.8.0_user_680767620.bin

Fire OS 6.5.5.5 (NS6555/4310)

https://d1s31zyz7dcc2d.cloudfront.net/4c4fd21537a97548236e9a88cd835030/update-kindle-biscuit_puffin-NS6555_user_4310M_0008087721594.bin

Fire OS 6.5.5.5 (NS6555/4313) - Contributed by phodina

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/994407ac6e55e2fb381de6f514555484/update-kindle-biscuit_puffin-NS6555_user_4313M_0008087722362.bin

Fire OS 6.5.5.6 (NS6556/4405)

https://d1s31zyz7dcc2d.cloudfront.net/2e17a264c416806ff7d3d984e1b6fcb7/update-kindle-biscuit_puffin-NS6556_user_4405_0008289072516.bin

Fire OS 6.5.5.8 (NS6558/4521)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/019175adb3adbdbd157f948fb74a471c/update-kindle-biscuit_puffin-NS6558_user_4521_0008624646532.bin

Fire OS 6.5.0.5 (NS6505/5041)

https://d1s31zyz7dcc2d.cloudfront.net/b221e0c9947ee49842251cc89c7b74ce/update-kindle-biscuit_puffin-NS6505_user_5041_0008960323972.bin

Fire OS 6.5.5.9 (NS6559/4657) - Contributed by Lemon86

https://d1s31zyz7dcc2d.cloudfront.net/61d2a62fa9d58c3184a13d23bb1b912c/update-kindle-biscuit_puffin-NS6559_user_4657_0009094443396.bin

Fire OS 6.5.5.9 (NS6559/4660)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/cd5c2a08fa0de682af56c9f7f14512b2/update-kindle-biscuit_puffin-NS6559_user_4660_0009094444164.bin

Fire OS 6.5.6.0 (NS6560/4774)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/6c94643670d218ad54d242c6ff16bc94/update-kindle-biscuit_puffin-NS6560_user_4774_0009295799940.bin

Fire OS 6.5.6.0 (NS6560/4779)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/a0db7929e0d3490e4762a79f3c08f09c/update-kindle-biscuit_puffin-NS6560_user_4779_0009295801220.bin

Fire OS 6.5.6.0 (NS6560/4781)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/55b25c2b40df952ceda06ed1cae3aa94/update-kindle-biscuit_puffin-NS6560_user_4781_0009295801732.bin

Fire OS 6.5.6.1 (NS6561/4929)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/6ce1a526b4763bc85917dd89dd0bd3be/update-kindle-biscuit_puffin-NS6561_user_4929_0009698492804.bin

Fire OS 6.5.6.1 (NS6561/4936)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/20f71463b25560a395bd375bd92356ea/update-kindle-biscuit_puffin-NS6561_user_4936_0009698494596.bin

Fire OS 6.5.6.2 (NS6562/5160)

https://d1s31zyz7dcc2d.cloudfront.prod.ota-cloudfront.net/2024/4/26/9ed9d91b-2bb8-417f-9eb1-7b0a61e510bd/update-kindle-biscuit_puffin-NS6562_user_5160_0010101205124.bin

edit: Included all the urls posted below to keep them in sorted order