Not every distro monitors/reports required restarts, so I imagine not everyone will even notice.

I’m not sure about Flatpak. I think the way Flatpak updates work makes it impossible for Firefox to update itself or detect that a restart is required, as the image doesn’t get patched immediately. My normal go-to is to scan for processes marked yellow or red in htop, but I don’t know if that works for Flatpak.

Unfortunately, Flatpak launcher integration is broken on both of my Linux installs so I barely use it.