From MozillaZine Knowledge Base
Firefox 3 (and later) and SeaMonkey 2.1 (and later) store their bookmarks bookmarks in places.sqlite and use JSON as the format for backups stored in the bookmarkbackups folder so, by default, bookmarks are no longer saved in HTML format. This preference allows bookmarks to be exported at shutdown to the Firefox or SeaMonkey profile folder as bookmarks.html. 
Possible values and their effects
Export bookmarks to bookmarks.html each time the browser shuts down.
Do not export HTML at shutdown. (Default)
First checked in
Has an effect in