At work our email servers are Microsoft Active Directory Exchange Servers. I use IMAP to access them. I also use Thunderbird for my IMAP mail client. I use the nightly builds and noticed that I was suddenly being prompted for a Kerberos login every time I sent email.
While I certainly could use Kerberos for the login, I prefer not to and only use standard IMAP and SMTP encrypted auth and not Kerberos. So, in order to completely disable Kerberos from Thunderbird I played around until I found the following combination. Just changing the using-native-gsslib to false will not work as I originally thought it would. I had to put the /dev/null in there to really get it to stop. All seems well for me now, MUCH happier!
1) Open Thunderbird Preferences
2) Advanced Button
3) General Tab
4) Press "Config Editor" button
5) Easiest to just Search for "gss"
6) There will be two entries that both need to be edited:
a) network.negotiate-auth.gsslib = /dev/null
b) network.negotiate-auth.using-native-gsslib = false
7) Close the Config Editor
8) Close the Prefs
9) Restart Thunderbird
Obviously be careful futzing around with the config. I hope this helps some folks!