Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0
I have also noticed that `delete-region` is not on any key. This could
mean that -- before Transient Mark Mode became default --
`delete-region` was intentionally kept out of easy reach. If this is
the case, was it because the earliest Emacs versions lacked undo?