They refuse to do it because the idea has absolutely no merit to it. If there's a virus on your computer that could steal your data, it can just wait till you unlock that data to steal it. There is zero practical benefit to implementing your suggestion.
"full-disk encryption" is the search keyword you're looking for
I'm a little confused as to how that would help with privacy/security.
When your browser is open and 'unlocked' a virus could still read the data.
It's the same thing with full disk encryption, if you get a virus on the running system it doesn't matter.
Isn't your computer disk encrypted already?
Otherwise you seem to want jails or sandboxes to protect each app, with access denied by default. That sounds more like Android, or possibly Qubes OS: https://en.m.wikipedia.org/wiki/Qubes_OS
Or flatpak
its open source, you can submit your patch... if they don't accept it, you can fork it
having a application try to make up for deficiencies of a operating system is a losing battle, better to isolate sensitive data at a container/vm level. i.e. Qubes, you can encrypt all the data at rest, and only unencrypt it when needed.
To protect it from ... what attack are you stopping here? If you don't know, and it sounds like you don't know, then forget it.
If someone roots your device, you still lose. If someone takes your device while you're browsing, you still lose. If your hard drive is unencrypted, you still lose.
Tired of seeing all these anti Firefox posts lately, especially when they instantly get debunked in the comments (which I am thankful for)
Is a local sandbox not an option for you?
Firejail on linux Sandboxie on windows
Chrome does lock some of your browser data to the current (windows) account. Which is why all chromium profiles aren't portable. You can't move your profile to another pc.
As for myself, I use an encrypted container for my ff profile. A fully encrypted disk is a better choice though.
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