File extension .BAK is used by Firefox Internet browser to provide a back-up for the bookmarks file of the consumer. Extension .bak is automatically created by Firefox to restore the bookmarks just in case the primary file becomes corrupted.

NOTE:The file extension must be changed back to the initial extension to ensure that the file to open right. This really is frequently done by merely removing the .bak part of the file name. For instance, my document1.doc.bak should be renamed to my document1.doc before being opened.

Once the Firefox software is installed in the machine, it will create yet another instance of the file bookmark.html and name it bookmark.bak.

Just in case the file bookmark.html gets corrupted, Firefox immediately retrieves the information included in the file bookmark.bak and renames it to end up being the brand new bookmark.html file.
The function of the file extension BAK is essential in securing data from within the machine.