Basically, the work to adapt NoScript and correspondingly expand the WebExtensions API has been going on for years, the original plan was to have a smooth transition when Firefox 57 came out, but there were some delays in the final days leading up to the release. Not recommended to stay on Firefox 56 because it won't get security patches; move to 52 ESR instead.
As far as I know, yes. Firefox has already decided to transaction away from XUL, so the extensions will break eventually. It's just a question of when the particular APIs your extension relies on get removed.
https://hackademix.net/2017/11/14/double-noscript/
Basically, the work to adapt NoScript and correspondingly expand the WebExtensions API has been going on for years, the original plan was to have a smooth transition when Firefox 57 came out, but there were some delays in the final days leading up to the release. Not recommended to stay on Firefox 56 because it won't get security patches; move to 52 ESR instead.