heres how i do it
make a file called startup.desktop
in ~/.config/autostart
make startup.desktop look something like
[Desktop Entry]
Exec=/PATH/TO/startupscript.sh
Icon=
Name=startupscript.sh
Path=
Terminal=False
Type=Application
and then you need startupscript.sh somewhere, which must be executable and look something like
# !/bin/bash
firefox &
sleep 1s
thunderbird &
another_command &
sleep 5s
etc &