Default Monitor for Applications to Open in?

Yes, this is a strange one. Here is the scenario:

- Two monitors side-by-side: HDMI2 and HDMI3
- Primary monitor is the monitor on the right (HDMI3)...think `xrandr --output HDMI3 --primary`

Now, when I open an application (say firefox), it wants to defaultly open in HDMI2 and *not* HDMI3. Is there a way to change this default behavior? I know it is a bit unusual for the right-most monitor to be the primary, but this is how I have my desk setup.

Thanks!