I think I messed up! My largest concern is not not being able to jailbreak again. It's rather this that my Kindle might be stuck at this version forever(or until there's a jb for 5.18.6 I guess).
I copied the the same latest firmware binary file to the root directory and restart the Kindle and it actually went through the process and showed "Update Successful" at the end. But I am still not sure if I will be able to apply the next kindle update.
Any help or info regarding this is much appreciated. Thanks.
Device: Kindle Paperwhite 12th Gen
Firmware: 5.18.6(latest, which does not support adbreak or winterbreak).