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
Back when Netscape had a mail client, it used a program called movemail to transfer new incoming messages from the user's inbox (at that time, that would have been an mbox spool I guess) to its own spool. I seem to recall that Emacs (RMAIL?) also followed this convention at some point. (In fact, now that I googled for a bit, Netscape apparently took this from Emacs, rather than the other way around.)
The "specification" was basically that Movemail would use some locking to make sure two processes would not attempt to fetch mail at the same time. As long as you always used the movemail program to fetch mail, you would not need to worry about its internals.
Jamie Zawinski's page is probably the most well-researched document you can easily find, although it is hardly authoritative: http://www.jwz.org/doc/movemail.html
Here is (a random, possibly mirrored copy of) the Emacs program: ftp://www.ai.mit.edu/pub/mav/netscape/movemail-src/movemail.c
Update: (A much extended) movemail is currently distributed as a part of GNU Mailutils