Backing up everything on my kindle fire with ubuntu before installing firefirefire, can't get past Hellions with blue flames when rooting
According to a post of by ch4lky in XDA:


  
    [-] Hellions with BLUE flames !
  
  
  This output means that the exploit didn't crash vold, or didn't know
  it did. The reasons can be :
  
  
  vuln[erability] has been patched
  no debug info in the logs
  no access to the logs
  the buffer size is different on your device than on the others (&lt;- can be fixed)
  


I did a quick read of zergrush's code; the log he's referring is located "/data/local/tmp/crashlog", you may want to look at that file to see if there is any useful info. 

Since according to the thread you linked to, the exploit has been reported to work for Kindle Fire, the most likely possibility is either the vulnerability has been fixed in the latest update (the vulnerability already had a patch in upstream since November) or you had a weird version that have different buffer size than other Kindle Fires.