Scrapbook, that takes me back. Haven't used it in a dog's age. But I would be enormously surprised if it did no longer store saved content under your Firefox profile directory, and back when I used it, it just saved the files there and maybe did some link rewriting. Not really a lot of importing necessary to view the content outside the extension - you'd just need to point a browser at its file:// URL.
Not sure how much that helps in terms of retaining the actual functionality, which unless I'm badly mistaken would only be feasible in the WebExtensions API via the external application messaging interface - and you'd need a separate program that would receive those messages and do the mirroring for you, and maybe expose a local HTTP server or some other such horrible hack to let you fetch the content tree for rendering in the browser as a table of contents/tree of bookmark-style links. But at least you might not have to lose what you've got.
Today I spent some time looking at how ScrapBook stores information. It's a bit of a mixed bag, with some plain text files, some XML, some HTML (besides the saved pages themselves). I managed to figure out how it stores folder structure, bookmarks, saved pages, and annotations. Fortunately there's no database, and no encryption, so I can write a tool to extract the information if needed.
The remaining question becomes: how to replace it? What other tool supports all this?:
* Local saving (as opposed to cloud)
* Storing source URL with support for re-fetching
* Bookmarks (for pages that won't save locally in a useful way, such as YouTube)
* "Deep saving," saving the main page AND linked pages, and keeping them bundled together
* Full text search
* Probably other features I do not recall offhand
Even if I can get the data out, it's hard to know where to put it. Most solutions these days are cloud-oriented, which is unappealing to me. I could build my own stand-alone replacement, but what a headache. I could fork ScrapBook and try to make it work with the latest Firefox, but I have no experience in the plugin domain, nor the time to prioritize learning it.
Sorry for the rant, I'm just trying to figure out how to proceed without severe productivity loss.
I'd look at the Zotero standalone, but that's just an offhand guess. Other than that, I got nothin' - except 52 ESR, which is good for security updates until some time next year, and won't get the breaking changes from 57.
Not sure how much that helps in terms of retaining the actual functionality, which unless I'm badly mistaken would only be feasible in the WebExtensions API via the external application messaging interface - and you'd need a separate program that would receive those messages and do the mirroring for you, and maybe expose a local HTTP server or some other such horrible hack to let you fetch the content tree for rendering in the browser as a table of contents/tree of bookmark-style links. But at least you might not have to lose what you've got.