First problem is that proper (incl. X11...) isolation costs you 3D (so not great for Firefox). Otherwise there is "toolbox" by Fedora, which you could surely combine with Xephyr for a wine-like, more isolated experience. I do that with a shell-only workflow (though on my HiDPI-machines I still have to patch Xephyr to scale the output (which is pretty easy using its GL-backend tbh, but I need to refine my amateur C dabblings to release that).

EDIT: I'm happy to discuss how to use Wayland with proper isolation and 3D! (and some frame-decoration)