I was going to also say that OP might be wanting something like Recall (which might be one of the few instances where it constantly saving shit would be perfect). But they would need like the most extreme version that isn’t just saving searchable screenshots.
I also think that one major issue for OP is more about how the actual sites are coded these days. As even if a single tab is being used, the shit can just decide to force it to update the contents at any time (like how just having Gmail open you will see new messages just show up even without refreshing your browser).
It seems like the perfect situation for OP would be if the web still worked like it did pre-web 2.0, but with using the current version of FF. Outside of that, it really seems like they need to just start having sites be auto-completely downloaded for full offline use.
I am still shocked that the main issues being had seems to be that it taking 10s of mins to allow FF to process that much stuff is the frustration. Which does seem to mean FF is holding up pretty well given the situation. Their complaint about tab isolation being too much overhead seems odd though. As it would seem that going back to not having that would mean a much higher chance of just everything just being yeet-ed out of nowhere.
I am not sure how their headspace of using virtual machines approach would be much better as shit would still have the issues of sites still self-updating and loading up in the first place. Though given they seem to have dramatically more coding experience, I am much more ignorant of this shit.
I also went completely to FF on my phone the moment it had uBlock Origin and some other extensions. Now that I learned how to force other ones, it has been game over for other browsers aside from just seeing how they are every now and then (I work with general public and need to be aware of how they work). The only thing I would really really advise in addition to FF on a non-rooted device is setting the Mullvad ad-blocking/tracking blocking DNS for the device.
It helps even if I need to use a different browser (not as good as also having uBO), and has really good chances of blocking ads in many apps. For example, I kind of treat the Microsoft Solitaire app since it keeps games and stats from my PC. However they have lots of video ads that play after a couple of games. With the DNS it just kind of glitches when an ad should run and just goes on to the next round instead. The only odd thing I see with other apps is that it can cause my bank app to take a little longer to load when signing in (I am guessing due to tracking it is trying to do). But after like ten seconds it goes through like normal.
This is the DNS address to add if you want:
https://base.dns.mullvad.net/dns-query
And here is the main site for all their options and tester to make sure whichever one you pick is working:
https://mullvad.net/en/help/dns-over-https-and-dns-over-tls