Mork (file format)

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Filename extension .mab, .msf, .dat
Developed by Netscape, later Mozilla Foundation
Type of format Database
Standard None (reference implementation)
Website Mozilla Developer Center

Mork is a computer file format used by several email clients and web browsers produced by Netscape, and later, Mozilla Foundation. It was developed by David McCusker[1] with the aim of creating a minimal database replacement that would be reliable, flexible, and efficient, and use a file format close to plain text.[2]


The Mork format was used in most Mozilla-based projects, including the Mozilla browser suite, SeaMonkey, Firefox, and Thunderbird. In Firefox, it was used for browsing history data and form history data. In Thunderbird, it is still used for many things, such as address book data (.mab files) and the mail folder summaries (.msf files).


Mork has many suboptimal properties. For example, despite the aim of efficiency, storing Unicode text takes three or six bytes per character.

The file format has been severely criticized by Jamie Zawinski, a former Netscape engineer. He has lambasted the ostensibly "textual" format on the grounds that it is "not human-readable",[3] bemoaned the impossibility of writing a correct parser for the format,[4] and referred to it as "...the single most braindamaged file format that I have ever seen in my nineteen year career".[4]

In response, McCusker stated that the problems with Mork resulted from "conflicting requirements" and that he merely fixed scalability issues in bad code he "inherited".[5]

The Register lambasted the Mork database with their article "Why has Thunderbird turned into a turkey?" [6]


The replacement system, used for storing all user configuration data, is called MozStorage. MozStorage is based on the SQLite database. Beginning with Firefox 3.0, Firefox uses it for its history, form history and bookmark data. The storage engine was also included in Firefox 2.0, but only for use with extensions.[7]

Mork was completely removed from Firefox in 2011.[8][9]

Plans existed for Mork to be replaced with MozStorage in Thunderbird 3.0.,[10] but the 8.0 release still uses the Mork file format. The current version of SeaMonkey, version 2.14.1, also still uses Mork for its indexes for both POP and IMAP mail folders (at least).

Software that handles Mork files[edit]

See also[edit]


  1. ^ "Mork What Is It — MDC". 2007-09-16. Retrieved 2009-08-19. 
  2. ^ "Mork Why — MDC". 2007-09-16. Retrieved 2009-08-19. 
  3. ^ Zawinski, Jamie. " Perl program to extract metadata from a Mork database". Retrieved 2009-08-19. 
  4. ^ a b "Bug 241438 – please make history.dat easier to parse (i.e., not Mork)". Retrieved 2009-08-19. 
  5. ^ "erys: resume: netscape: mork: jwz". Internet Archive. 2005-05-25. Archived from the original on 2005-05-25. Retrieved 2009-08-19. 
  6. ^ "Why has Thunderbird turned into a turkey?". 
  7. ^ Paul, Ryan (2006-04-30). "Features cut from Firefox 2". Retrieved 2009-08-19. 
  8. ^ "Remove Mork from Mozilla-Central". 
  9. ^ "Mork to be removed from Firefox 7". 
  10. ^ "Thunderbird 3 Planning". 2009-08-10. Retrieved 2009-08-19. 

External links[edit]