This in combination with "Restore all tabs on startup" not being the default is a disaster.
The first thing I do on a new Firefox installation is enable "Restore all tabs on startup".
Even without that flag enabled, you can restore all tabs when closing/reopening Firefox.
History > Restore Closed tabs.
Thanks, good to know.
This is in all apps in OS X, that's the standard keymapping.
https://bugzilla.mozilla.org/show_bug.cgi?id=52821