It would be nice to have a hook for when a buffer was going to be killed, but before
the xul:browser had been destroyed.  Not saying that "kill_buffer_early_hook" would
necessarily be the best name... would have to look at other hook names and try to be
consistent.  Also, this hook should be a coroutine hook so that people can use it to
have a buffer-kill confirmation.

Such a confirmation prompt might actually be more appropriate in the kill-buffer command.
Things to decide.