before going through the 5.4.0 downgrade procedure I updated to 5.4.3.1 to give it a try...first impressions surely better than 5.4.2.1, I'll keep it for now.
Jailbreak survived.
After that I was able to log (and block) this bastard through my router:
after a reboot it keeps contacting the following 2 ip addresses (forever) which are third party unknown addresses (maybe amazon has some mirrors local servers housed at those providers)
PHP Code:
IN=br0 OUT=ppp0 SRC=192.168.1.106 DST=212.52.97.15 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=44201 DF PROTO=UDP SPT=36351 DPT=53 LEN=40
IN=br0 OUT=ppp0 SRC=192.168.1.106 DST=193.70.152.15 LEN=60 TOS=0x00 PREC=0x00 TTL=63 ID=44401 DF PROTO=UDP SPT=44898 DPT=53 LEN=40
Without letting these packets through the firewall the wireless appears disconnected on the kindle (no wi-fi icon and signal bars empty)
I set the wireless off after that and kept the router drop rules, new firmware never had a chance to go out on the internet yet.