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.
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.