After a long confusing research I'm wondering what format specification the movemail account type in Thunderbird (version 24) is referring to. As far as I'm concerned, movemail is a program to move Mail between mbox files (http://mailutils.org/manual/html_node/movemail.html#movemail), but not a specification. There're a lot of tutorial and forum threads on the topic (see below), but none of them mentions versions or references specification in a useful way. Following release notes is difficult because they're only rarely explaining impact of implementation changes. Documentation from the side of Thunderbird seems to be missing completely (latest is the wiki https://wiki.mozilla.org/Thunderbird:Help_Documentation:Creating_an_Account#Creating_a_Movemail_Account where screenshot refer to version prior to 12 if I recall correctly).
My research result so far:
http://ubuntuforums.org/showthread.php?t=1718795
https://askubuntu.com/questions/301988/using-movemail-with-thunderbird-on-ubuntu
http://forums.mozillazine.org/viewtopic.php?t=11772
https://askubuntu.com/questions/301988/using-movemail-with-thunderbird-on-ubuntu (involves postfix which shouldn't be necessary)
http://forum.ubuntuusers.de/topic/fetchmail-procmail-thunderbird/ (German, google translate: http://translate.google.com/translate?hl=en&sl=de&tl=en&u=http%3A%2F%2Fforum.ubuntuusers.de%2Ftopic%2Ffetchmail-procmail-thunderbird%2F), strongest indication for mbox format
https://askubuntu.com/questions/1916/how-can-i-access-system-mail-in-var-mail-via-thunderbird