It will remain updated through April 2018 and your profile will stay separate from your installed copy of Firefox. In April, ESR is switching to a newer Quantum version of Firefox, so old extensions will be disabled. If, at that point, there isn't a suitable replacement for ScrapBook, disable updates in your copy of Firefox Portable (if you're using the PortableApps.com Platform to automatically update it, rename your FirefoxPortableESR folder to FirefoxPortableESROld or similar) and only use it for ScrapBook not to go online.
You can 'install' it anywhere you want (Documents folder, other drive, cloud folder, etc). You can copy your profile in using the steps outlined here: https://portableapps.com/support/firefox_portable#local_prof...
It will remain updated through April 2018 and your profile will stay separate from your installed copy of Firefox. In April, ESR is switching to a newer Quantum version of Firefox, so old extensions will be disabled. If, at that point, there isn't a suitable replacement for ScrapBook, disable updates in your copy of Firefox Portable (if you're using the PortableApps.com Platform to automatically update it, rename your FirefoxPortableESR folder to FirefoxPortableESROld or similar) and only use it for ScrapBook not to go online.