From MozillaZine Knowledge Base
bookmarks.bak is a Firefox backup of bookmarks.html in case of computer failure (ex: power outage). When Firefox is started and bookmarks.html is either missing or reduced to "0", bookmarks.bak will be used, if present, instead of the default set of bookmarks .
This file can be edited in the same manner as bookmarks.html, but there's no reason to do so.
This file can be moved to a different profile without any extra effort, but there's no reason to do so.
Deleting bookmarks.bak will have no effect except for losing a backup. The file will be regenerated, normally the next time Firefox is closed.
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