firefox & multi-desktop

Hi,

i'm wondering why firefox doesnt work properly with multi-desktop.

I have 4 desktop (in osx, not a reel screen), usually, i can open an app on a desktop and it stay there, i can move from one desktop to an other.

But with firefox, the firefox window is open on every desktop.

Due to that, the multi-desktop become usless

How can i manke firefox to stay on one desktop ?

PS:

firefox version 44

OSX version 10.11.2

macbook air 13" 2013

if you need any other information, i'll be happy to give you them

thanxs