From MozillaZine Knowledge Base
The bookmarkbackups folder contains bookmarks-(date).json files in Firefox 3.0 and above (daily rotating backups of bookmarks stored in places.sqlite) or, in Firefox 2 and below, bookmarks-(date).html files (daily rotating backups of bookmarks.html).
This folder can be moved to a different profile without any extra effort.
Deleting this folder will cause the loss of all bookmarks backups. It will be regenerated when it is needed.
Related files and folders
Related bug reports