From MozillaZine Knowledge Base
Firefox 1.5 introduced a dated bookmark backup feature to compensate for the relatively common case of bookmarks being lost due to a system crash. By default, five sets of bookmarks are kept in the bookmarkbackups folder. However, there are situations where a user may want to disable bookmark backups, or increase the number of backups kept. This preference allows for those options.
Possible values and their effects
A non-negative integer corresponding to the number of bookmark backups to keep. Setting to 0 will disable backups; -1 will keep an unlimited number of backups. The default is 5 in Firefox 3 versions and has been increased to 10 in Firefox 4+ versions .
First checked in
Has an effect in