I shut down Firefox overnight by running killall -s SIGTERM firefox via cron, in order to apply the package manager and browser addon updates. Unfortunately each time when the browser is (manually) restarted, a 'restore session' tab about:sessionrestore is opened, obviously indicating that Firefox doesn't consider this as a clean exit. I'm relying on the 'delete cookies and site data when Firefox is closed' feature (with some domain exceptions) which supposedly has the potential to misbehave when the Firefox processes are killed.

The Mozilla documentation doesn't mention any such feature.

Any Wayland-friendly solutions (including, potentially, Selenium and some sensible macros) are also accepted.