What is .BAK file?
Backup of the Firefox bookmarks file; can be used to replace saved bookmarks in case the main bookmarks file becomes corrupted; automatically generated by the Firefox Web browser; typically named "bookmarks.bak"
The bookmarks.bak file is located in the following directories on Mac and Macintosh systems:
C:\Documents and Settings\UserName\Application Data\Mozilla\Firefox\Profiles\[randon-name]
Recommended Software Programs to open .BAK file