While most Flatpaks by default aren't all tuned up, you can easily set overrides for them, see `man flatpak-override`. There also exist `Flatseal` which is a GUI for doing so.
AppImage can be used with Firejail.
Firejail use is trivial to setup, we try to keep out of box breakage minimal but still strict, just `sudo apt install firejail && sudo firecfg` + a logout and back in.
Disclosure: I am one of the core Firejail contributors.
AppImage can be used with Firejail.
Firejail use is trivial to setup, we try to keep out of box breakage minimal but still strict, just `sudo apt install firejail && sudo firecfg` + a logout and back in. Disclosure: I am one of the core Firejail contributors.