Mork
From Just Solve the File Format Problem
Mork is a Netscape/Mozilla database file format used in various programs (such as the Firefox history file in version 2). Its use has been replaced by other things (primarily SQLite) in some of the more prominent Mozilla uses these days, but it is still used in the Thunderbird e-mail program (according to Wikipedia, though Chris Blizzard reported in 2011 that it had been excised from the entire Mozilla code tree).
It is text-based, but not very human-readable. Former Netscape engineer Jamie Zawinski has called it "...the single most braindamaged file format that I have ever seen in my nineteen year career" (written in a Mozilla Bugzilla report in 2004).
