By default Linux Mint XFCE opens programs on the monitor they were last present.
This doesn't work for some programs like Mozilla Thunderbird although it is reported that if these programs are unmaximised and moved to the correct monitor and then closed they remember their position on opening.
I liked the way the i3 window manager enabled one to set the position of programs.
In order to move a maximised no title-bar Thunderbird it is necessary to unmaximise it temporarily.
Requires wmctrl