See Mozilla and Data Store. Written by Dave McCusker. Actually not a db, but a text format. MDB was an abstraction that provided an interface for various Mozilla modules (Address Book-s, BookMark-s, etc.) to call, so they wouldn't have to worry about how the underlying Data Store was implemented (which had been a problem with previous releases being overly integrated with another specific db, BerkeleyDB I think).
Some other links:
http://tln.lib.mi.us/~amutch/pro/kmeleon/khistfiles.htm (links to mork.dll)
I'm not quite sure of the relevance of http://www.treedragon.com/ged/mc/in/lh19Mar00.htm
User data is stored (in Win98) in c:/windows/application data/mozilla/profiles...
address book (for imap email) is named 'imapab.mab';
data looks like '(99=Matthew Adkins)(9A=Adkins)(9B=Matthew)(9C =email@example.com$0A+81-78-691-2719 in Japan)(9D=Chicago) (9E=IL)(9Ffirstname.lastname@example.org)' (in my case, data starts with cellID=81, and continues through one record at a time, with the cellID incrementing continuously)
header (defines field names with fieldIDs) looks like '(85=DisplayName)(86=NickName)(87=PrimaryEmail) (88=LowercasePrimaryEmail)(89=SecondEmail)(8A=PreferMailFormat) (8B=WorkPhone)(8C=HomePhone)'
footer (maps cells to field IDs) looks like '7(^85^99)(^84^9A)(^83^9B)(^A8^9C)'
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