In case anyone else is automating their Firefox + puma-dev setup, here's one way to install the puma-dev CA cert without using the GUI or other manual intervention:
# Tell Firefox about the puma-dev CA cert. Don't care whether Firefox is
# currently installed, since we still need this to work when/if it is.
#
# https://github.com/mozilla/policy-templates/blob/master/README.md#certificates--install
for firefox in firefox firefoxdeveloperedition; do
plist="/Library/Preferences/org.mozilla.$firefox.plist"
[[ -f "$plist" ]] || sudo plutil -create xml1 "$plist"