How can I force fullscreen on my right screen?

I have 2 screens: 17" on left and 22" screen on right side.

When I watch a movie with VLC and want to watch on full-screen on my 22" screen (the right one), I move my VLC window in right screen and then click on full-screen => no problem.

BUT when using Firefox and watching streaming video, watching on full-screen is always on 17" screen (left one).

How can I force full-screen to be on my 22" screen (right one) ?

After reading severals questions about the subject "multiple-monitors", I've found the solution.
In this answer I've discovered the existance of file ~/.config/monitors.xml. (thank you "pamar84")

So I did this:

1. do a backup of this file cp ~/.config/monitors.xml ~/.config/monitors.xml-bkp
2. open this file gedit ~/.config/monitors.xml
3. this file was containing 3 configurations !!!
   So I did some cleaning:
   • I've found the configuration that I'm using and remove the 2 others.
     
   • also set my left screen to be the primary one.

Now with such nice ~/.config/monitors.xml file, Firefox is not disoriented anymore and have the good behavior:
- when my window is left side and I choose full-screen => full-screen is left screen
- when my window is right side and I choose full-screen => full-screen is right screen

Here is the content of the cleaned ~/.config/monitors.xml file: