From MozillaZine Knowledge Base
Since Firefox 2 and Firefox 3 use different backends to store bookmarks, there can be issues switching back and forth between the two versions while using the same profile. This preference helped guard against dataloss by letting users set the automatic bookmark import/export behavior. It was removed when JSON became the default bookmark backup format (in bug 384370).
Possible values and their effects
When importing bookmarks.html into Places, make a copy as bookmarks.preplaces.html. Each time the browser quits, export the Places bookmarks as bookmarks.html.
Do not make a copy when importing bookmarks.html. Each time the browser quits, export the Places bookmarks as bookmarks.postplaces.html. (Default)
First checked in
Has an effect in