Standard notes is available as appimage from their website, as a snap on ubuntu and also as flatpak on flathub. How did you install or run it? Also, please don't equate ubuntu with linux, unless you had troubles installing it on other distros as well. It scares away other people trying to move to good gnu+linux distros (like linux mint, fedora, etc).
AFAIK running firefox in a terminal and pressing ^C (SIGINT) has kind of the same effect as logging out or poweroff in GNOME (SIGTERM, if you're using systemd). This gives the browser (or other processes with crash recovery) enough time to save all its data and exit gracefully for the crash recovery the next time they are run.
The confirmation is annoying for many GNU+Linux users. It's like asking are you sure you want to power off even though you had to use three or four keys or mouse clicks just to get to the poweroff menu.
Nice! The UI looks like Amberol