Do you mean sandboxed?

Unfortunately Mozilla is going the enshittification route more and more. Or good in this case that the Firefox Phone did not take of.

Nice. Have fun with it!

Agreed. My answer was in the assumption that it’s about desktop apps as the OP mentioned Electron.

I think I heared that lan-only mode means you need to transfer the files to print via SD card.

Go with one of the ready to use systems. Flatpak, Snap, AppImage. Snap is largely Ubuntu Ecosystem, Flatpak is independent. AppImage is an option if you do not need/want a Sandbox.

Stay away from Docker and LXC for this use case (graphical applications), they are much more work to get going.