/etc/xdg/xdg-xubuntu/xfce4/helpers.rc is in xubuntu-default-settings 18.04.6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | WebBrowser=firefox
MailReader=thunderbird
FileManager=Thunar
TerminalEmulator=xfce4-terminal
|