Another option would be a sandboxing system like xdg-app. AFAIK, there's already a xdg-app release of LibreOffice, and an experimental xdg-app build of Firefox.
And, of course, there's always the paranoid option of "run the browser within a virtual machine", so an attacker would have to break the browser, then get local root within the VM, and finally find an exploit for the VM, before getting to your files.
And, of course, there's always the paranoid option of "run the browser within a virtual machine", so an attacker would have to break the browser, then get local root within the VM, and finally find an exploit for the VM, before getting to your files.