Creating ~/.XCompose causes Emacs, URxvt, and xterm to ignore the default /usr/share/X11/locale/${LANG}/Compose, while Firefox and KiCad read both.
At first I thought it was just Emacs being different, and doing something wrong because NIH.
But after consulting the Compose(5) manual, it's Firefox and KiCad that are wrong for reading both (I should put include "%L" in my ~/.XCompose to include the system file). Probably they both inherit this bug from #GTK (and Emacs' NIH saved it).