What I would like instead is a browser that treats tabs more like virtual machines that you can roll back, suspend to disk and resume. Little package of data that get frozen in time and are externally searchable.
Maybe look at ArchiveBox. IIRC it has pretty much everything you ask for including an import from your browser history and bookmarks.
I’m not going to tell you that you’re managing your information wrong. I would physically die if I had ever more than 20 tabs (my ADHD couldn’t handle it).
But I think you might be using the wrong tool. A browser (like Firefox) is not really designed as an information manager. It’s primary purpose is navigating and visualizing web pages. So when you talk about “a few megabytes of text and images” thats not what your browser sees. Your browser handles more than just the text and images. It also handles fetching and prefetching, a browser history for every tab, a JS context and much much more.
What you want is some kind of personalized archiving system that processes websites into machine processable (ie searchable) structures. Firefox is not that. Maybe data hoarder communities will have the answers you seek.
If every country did as great as the UK per capita we would be completely and irrevocably fucked.