Whatever rules Mozilla Firefox follows to determine the user’s default email client don’t seem to work properly on Debian, at least not when GNOME is not the desktop environment originally installed. For whatever reason, what Firefox tries to do with
mailto links and the Send Link context menu option is to open the Evolution mail client, which is not installed with the KDE or LXDE desktop environments.
Fixing this is supposed to be trivial, and it is, but the relevant option is hidden in the worst possible place in Preferences.
Chromium/Google Chrome does not have a user-accessible setting for this, but somehow still appears to get the right idea from the desktop environment.