check your Firefox profile folder in c:\documents and settings\<your username>\application data\mozilla\firefox\profiles\<profile id>
you may find backups of your bookmarks there.
Look for a bookmarks.bak, bookmarks.html.sbsd.bak or a boomarkbackps subfolder. Depending on your version of Firefox these files or folder exist and contain backups of your bookmarks.
This
article may help you importing them.