BAK (Firefox Bookmarks Backup, Backup File)
The BAK Misc file format
In general terms, the BAK file extension is simply a backup file. Many different types of programs protect data by creating a BAK file. This typically happens automatically when you (or the program) are saving the file or when you are exiting the program. In these scenarios, the program first creates a backup file that contains the information from the old file and saves it with the BAK extension. Then it saves the original file in the original location. This is typically done so, in case the current file becomes corrupt, the previous file is still available and all the information can be recovered.
Technical details of BAK files
If you are having a problem with a file, in most cases, manually changing the file extension will not solve it because different types of files are set up differently and have different functions. However, in the case of a BAK file, that is not always the case. Since the BAK file is a mirror of the original file, in many cases if you are having a problem with the original file, you can simply locate the BAK file, change the extension to match the extension on the original file and replace it. Mozilla Firefox is one of the programs that, most typically, creates BAK files. It is set up to automatically backup your bookmarks.