Replacing /usr/bin/firefox doesn’t fix anything if you don’t restart Firefox itself.

On my box updating firefox and then restarting it won’t even launch the new version because NixOS knows I’m logged in and won’t just change things in my environment. But unless there’s a kernel update yes nixos rebuild switch followed by logging out and logging in is equivalent to rebooting as it will automatically shut down and restart all system services, I think even systemd itself. Modulo some wibbles around kernel modules but those fall under kernel updates in my book.

Contrast Ubuntu, which really likes to prompt your for reboots. The difference between a distro primarily for desktop use and one that can also do desktop because also devops want a desktop. Hey I could spin up 1000 cloud instances of my desktop with a couple of keystrokes isn’t that impressively useless :)