ScrapBook AutoSave Plus is a modified version of the original ScrapBook Add-On AutoSave, which you find here: http://amb.vis.ne.jp/mozilla/scrapbook/addons.php?lang=en#AutoSave
This version will work with ScrapBook Plus or with ScrapBook with XSidebar for Seamonkey but not with ScrapBook version 1.4.x or higher because of API changes in the original ScrapBook version. So don't use this add-on if you have installed ScrapBook version 1.4.x. Instead of ScrapBook version 1.4.x use ScrapBook Plus version 1.8.18.x.
You find ScrapBook for Seamonkey with XSidebar here:
You find ScrapBook Plus for Firefox here:
Before you can use this add-on you have to uninstall the original extension AutoSave. Don't enable both extensions AutoSave and AutoSave Plus at the same time!