Social and technological hurdles for development were big enough that I never pursued it past that. One issue is that the Firefox product inside Mozilla was starting on a trajectory towards becoming increasingly inward-focused, disfavoring ideas that didn't come from within, and following a more corporatized product development approach. So what one would need to do (and ideally everyone working on new, discrete features should be doing, anyway) is to start out by building it as an extension to work as a proving ground.
The problem there is, in order for that to work in any sort of reasonable way, changes to Firefox itself would need to be made (APIs reworked) so that the surface exposed to you there is not one that's running orthogonal to what you need. So in order to develop the thing, you'd need to get some cooperation to get these non-trivial core changes upstream. GOTO 10
AFAIK (I used to be on top of a huge part of what came out of the Mozilla firehose, but I don't do any of that anymore), the whole situation is no better today (and really, from what I understand, much worse). Chromium might be better there, but I really have no idea.
And for local storage, check out https://webrecorder.io/ for an example implementation.
There's definitely a niche in the market for a bookmarking site that does some form of bulk importing of your bookmarks; has really easy organisation (drag and drop, and an API for power users); has some kind of thumb-nail; has some kind of link to Internet Archive (to read old sites); and maybe a link to IA to store sites. The internet archive stuff could be a paid option with some money going to IA to help fund them.
Why?
The only solution to this problem I found is storing the links locally. I'm now in the process of importing everything into OneNote (onenote clipper is a huge help). A big plus is, that the content is indexed and fully searchable.
I probably would not do this if internet archive was more reliable. I'm ok with this solution, but it's a bit strange that firefox/chrome/IE haven't made this process of storing sites locally easier.