Right but that doesn't happen quite often either. Also if it does how important is restoring the tabs and windows? Probably not to a lot of people other than serious creators that use a web editor of some sort that'll lose work if the browser disappears. Even then it is flaky to rely on something like this. Much better to have periodic backups of whatever you're doing.
You say "doesn't happen quite often", which I'm assuming means something like "doesn't happen quite often to me". But if you consider hundreds of millions of users using Firefox every day, then even a relatively rare event is happening to multiple users every single day. It is absolutely worth it to them that Firefox does its best to preserve their data.
